Abstract: Investment is regarded as one of the most fluctuating components of total demand in the economy. The purpose of this study is to investigate the influence of the fluctuation in investing on the commerce business periods in the economy of Iran. The data used in this paper are of chronological type related to the period between 1983- 2006. In this study, after de-processing the chronological series of all variables in the target model, using filters in a press-cut, and considering correlation coefficient among variables, the co-relation among them has been analyzed. In the next step, to investigate the effects of Gross Permanent Investment on commerce business periods in the economy of Iran, a model in the frame of simultaneous equations system has been utilized which has been analyzed using the method of 3-stage squares minimum. The results showed the variable of investment has positively effected on the commerce business periods in the economy of Iran, and this is in accordance with the theory of commerce business periods.