In recent years J2ME(Java 2 Platform Micro Edition) has been wildly applied in cell phone market. Therefore, J2ME’s ability to work with different platforms is important in the fragmented cell phone market. This capability lets a Java-enabled phone run applications and services written for other mobile platforms and also lets software vendors save time and money by writing a single, Java-based version of an application to run on multiple platforms. J2ME platform provides KVM(Kilobyte Virtual Machine) , which is an implementation optimized for use on small devices. However, limited memories constraints make JIT techniques untenable. Although it can compile java source code to native machine code for performance, it will loose the portability. We have developed a Java mixed-level compiler for compiling Java high-level language and JAsm(Java Assembly Language). The programmer can use mixed-level language to optimize original source code. The compiler translates mixed-level source code to mixed-level syntax tree. Then, it transforms mixed-level syntax tree into Java assembly code, and writes out the class file. Based on the experimental results, it can be seen that our approach can achieve better performance. Consequently, the programmer can use JAsm to optimize java source code.