Hardware software codesign using various hardware and software implementation possibilities requires a cosimulation environment which has both flexibility and efficiency. In this paper, a hardware software cosimulation environment is developed using the backplane approach and optimized synchronization. To seamlessly integrate a new simulator, this paper defines and implements the backplane protocol for communication and synchronization between client simulators. Automatic interface generation facility is also devised for more effective cosimulation environment. To enhance the performance of cosimulation backplane, a series of optimized hardware software synchronization methods are introduced. Efforts are focused on reducing control packets between simulators as well as concurrent execution of simulators without roll-back. The environment is implemented based on Ptolemy and validated with a QAM example run on different configurations. With optimized synchronization method, we have achieved about 7 times speed-up compared with the lock-step synchronization.