In conventional cyclic steam stimulation (CSS), steam is periodically injected at high pressure to the reservoir. After the steam injection period, the well is soaked for several days then it will be produced. Generally, for the first to the fifth cycle, the steam can effectively transfer the heat to the reservoir. After that, the CSOR will rise up indicating that the process is currently ineffective. This paper aims to improve the CSS performance using modified well completion. The perforation is modified to become two parts, one part is on the top side (as injection) and the other part is on the bottom side (as production). In this process, after being injected, the steam will condense, resulting from heat loss, and it will move to lower part because of gravity drainage. Simultaneously, crude oil was produced through the production perforation. The opening-closing of the injectionproduction cycle is managed by interval control valve (ICV). To provide an overview of this phenomenon, a synthetic reservoir model was built based on Pertama-kedua formation, located in Sumatra Indonesia. Sensitive variables are length of the injection-production perforation and soaking time. Finally, the heat efficiency was evaluated during 8 years of project life. Simulation results show that dividing the perforation into injection and production intervals will reduce CSOR 30% and this requires shorter soaking time compared to that of conventional processes. Furthermore, if the distance between injection and production interval is longer the production will be better. However, this gap is limited by reservoir thickness.