Behavior of a chemical reaction system for converting methanol to light olefins in a fixed bed reactor under a periodic operation was investigated via numerical simulation to improve the efficiency, of the system through the enhancement of yields of the desired products such as ethylene and propylene. In the periodic operation, the reactor wall temperature was set to vary sinusoidally along its axis. The recycling of by-products such as pentene and butylene was also studied. As a result, we found that the combination of the wall temperature modulation and the rec-cling of pentene led to the possible increases in the ethylene and propylene yields Of LIP to ca. 33% and ca. 16%, respectively,, compared with those obtained with a uniform wall temperature. Further, it was shown that the product ratio of propylene to ethylene can be changed in the range of 1.60 to 6.05 when butylene is recycled, This suggests that production with variable composition of ethylene and propylene according to need is possible by changing the wall temperature modulation and the recycling ratio even in the same reactor.