The purpose of this final project is to determine the effect of environmental stimuli of store on the behavior of unplanned purchases in Alfamart Sisingamangaraja Semarang. This research was conducted in the city of Semarang; the sample taken were 100 people. The methods of data collection used in this research were interview, literature study and questionnaires. The questionnaires use a semantic differensial with 10 point scale and the sample was selected by incidental sampling. The data analysis technique used was Simple linear regression. Based on the calculations using IBM SPSS 20.0, the t test result (7.132) was bigger than t table (1.984) and had a significance (0.000) less than significance level of 0.05. So, it means that the environmental stimuli of store had a positive and significant effect on the behavior of unplanned purchases. The result of R square is 34,2% had a meaning that 34.2% of the behavior of unplanned purchases (Y1) could be explained by changes in the environmental stimuli of store (X1) variable, while the remaining of 65.8% was explained by other factors which are not observed in this study.