The aim of the study was investigation of the effect of twice application of the Group Investigation method at different intervals on improvement of the students' academic achievement and science process skills in teaching the science lesson. Method of the study was a quasi-experimental design. The study used Solomon Four-group Experimental Design in the first year, and an experimental design in the second year which was created by adding a new experimental group to involve three experimental and two control groups. It was applied the Group Investigation method in the experimental groups and current methods on the curriculum of the secondary school science curriculum of Ministry of National Education in the control groups. Data collection process were used qualitative interview, preliminary information, science process skills and academic achievement tests. Applying the Group Investigation method for two years proved positive contributions to students' academic achievement and science process skills in science lessons. The researchers believe that experimental model developed this study will be used in different research areas in long-term studies in the future, and these studies will also make a great contribution to the literature.