Readiness of CMS Simulation Towards LHC Startup

CMS experiment has used detector simulation software in its conceptual as well as technical design. With the detector construction near its completion, the role of simulation has changed toward understanding collision data to be collected by CMS in near future. CMS simulation software is becoming a data driven, realistic and accurate Monte Carlo program. The software architecture is described with some detail of the framework as well as detector specific components. Performance issues are discussed as well