The experimental study investigated the effect of science activities (‘Let’s Think!’) on the development of thinking skills of early school children in Pakistan. A total of 101 experimental and 130 control group children of grade 1 participated in the study. The sample comprised 4 experimental and 5 control groups was selected from 6 different schools. The mean age of children was 83 months (SD=11.4) at the time of pretest. Thirty science activities were conducted as intervention for a period of 1 year. The Schemata i.e., Seriation, Classification, Time Sequence, Spatial Perception, Causation, Theory of Mind and Concrete Modeling were addressed in the activities. For measuring the cognitive development of children, drawing and conservation tests were used as pre and posttest. The purpose of drawing test, that intended to measure the schema of spatial perception, was to see the effect of science activities on the schema addressed in the intervention program. The conservation test was used to examine the effect of science activities on the schema other than those addressed in the intervention program. Independent sample t-test was applied to see the difference between the mean gain scores of the experimental and the control group. Hedges’ g effect size was calculated on the gain scores to measure the magnitude of effect on the thinking skills of children. The results of the study showed that the gain scores of the experimental group on the drawing and the conservation tests were significantly higher than those of the control group. The effect of intervention on the drawing and the conservation tests was significant both in boys and girls. However, the values of the effect sizes on both of the tests were higher in the girls than the boys. When age-wise comparison was made, it was observed that the effect of intervention on the drawing test was significant at age 6 + , 7 + and 8 + years. In case of the conservation test, however, it was significant at the age 6 + and 7 + years. The value of the effect size on conservation test was the highest at the age 6 + , which dropped gradually in the age 7 + and 8 + years. Furthermore, the effect of intervention on the drawing and the conservation tests was significant in the girls at age 6 + and 7 + years, whereas for the boys, it was significant at the age 6 + only. Phases of rapid brain growth iiand plateau and the gender differences in these phases give an account of the differential effect of intervention on age and gender. The results of the study suggest that thinking skills intervention programs aiming at the development of general thinking ability of children should be introduced in schools at appropriate time.