Investment behaviour comes under the field of behavioural finance. It is a process of investors' decision making at micro level. The process based on some specific features of investment products and services. This study was subjective in nature and cleared the scope of quantitative research in this regard to understand the investment behavior of generation Y investors while they are investing in the stock markets. A purposive sampling technique was used to collect data from the 325 respondents representing generation Y working in financial institutions. A structured questionnaire technique was used for data collection so the analysis could reflect true results. Multiple regression analysis technique was used to test the study hypotheses. The findings of this study indicates that investment choices, level of risk and behavioural biasness have direct influence on the portfolio selection of the generation Y investors and they are not behaving according to traditional theories of finance. The results also demonstrate that the generation Y investors of financial institutions understand the stock selection criteria and perceived risk attitudes toward the investment decisions. Theoretically, this study provides a possible solution to understand the behaviour of generation Y investors. Practically, this study is helpful for financial institutions to suggest best financial securities according to the behaviour of generation Y investors. At the end, this study also contributing in the existing literature on the behaviour of generation Y investors.