Chillies (Capsicum spp.) are the second largely consumed vegetable in the world and belong to the family Solanaceae, also known as the night-shade family. Chillies require 20- 30 °C optimum day temperatures for growth and development, usually growth is retarded under below 15°C or increase above 32°C temperature. Various stages of Chilli plant are affected by high temperature which leads to economic yield losses in fruit production. The present study was designed with four experiments to evaluate the various morphological, physiological and biochemical attributes that can be used for evaluation of heat tolerance potential in chilli genotypes. Research was conducted in growth room of mushroom lab at Institute of Horticultural Sciences, University of Agriculture, Faisalabad. In the 1st experiment, comparative performance of 25 chilli genotypes was observed under high temperature stress (40/32°C day and night, controlled conditions). Various morphological attributes were recorded and showed significant differences among genotypes. All the genotypes were categorized according to their performance under high temperature stress. Out of 25 genotypes, two genotypes C-37 and uk-101 were found to be most heat tolerant while other two genotypes H-13 and Jawala were found to be most heat sensitive. These four chilli genotypes (heat tolerant and sensitive) were used for further investigations. While in the 2nd experiment 4 chilli genotypes (two tolerant and two sensitive screened out from 1st experiment) were selected for further investigations under high temperature (40/32ºC day and night temperature) in growth room. Different morphological, physiological parameters, water relation attributes and biochemical attributes were recorded. In this study tolerant genotypes C-37 and uk-101 showed better performance as compared with heat sensitive genotypes (H-13 and Jawala) under high temperature stress. In the 3rd experiment, same 4 chilli genotypes, two tolerant (C-37 and Uk-101) and two sensitive (H-13 and jawala) screened out in Experiment # 1 were selected and sprayed with different levels (0, 5, 10, 15 and 20mM) of glycine betaine at the seedling stage under high temperature stress (40/32ºC day and night temperature) in growth room (controlled conditions). Present research was designed to find the role of glycine betaine in inducing the heat tolerance in chilli genotypes and also find the best glycine betaine dose that is more useful under high temperature conditions. Different attributes of chilli genotypes were recorded. Results revealed that glycine betaine application @15mM is best for enhancing the heat tolerance potential of chilli genotypes. Fourth experiment is a further study of experiment #3 in which optimized dose (@ 15mM) of glycine beatine was used on chilli genotypes to check the deleterious effect of high temperature stress (40/32ºC day and night temperature). Different morphological, physiological attributes, water relation attributes, biochemical parameters and yield parameters were recorded. It was observed that exogenously applied optimized dose (@ 15mM) of glycine betaine exhibited significant effects on all the chilli genotypes as well as improved the yield by improving heat tolerance potential of genotypes under high temperature stress. It can be concluded from the present research, indentified heat tolerant chillli genotypes are helpful in extending growth period of chillies. The optimized dose (@ 15mM) of glycine betaine has also proved effective in extending the growth period of chilli by enhancing the heat tolerance potential under high temperature stress.