Abstract: The Java virtual machine is studied, which is the core of a Java platform. In order that Java can run smoothly on various platforms, the composition and running mechanism of the Java virtual machine are studied and a reference for implementing JVM and porting JVM to other platforms is provided. Through an analysis on the source of Kaffe which shows a case of implementation of the JVM, the flow chart of a running JVM it summarized and in some depth the JVM’s key parts are studied. At the same time, some key steps in implementing JVM are summaried and some advices are given which can be used to improve the running efficiency of JVM.