This is because you have not set the environment variable. Right-click My Computer-Properties-Advanced-Environment Variables, find Path in the system variables, and add the bin directory of your JDk at the end. For example, my computer is in: C:\ProgramFiles\Java\jdk1.6.0_26\bin, then you add;C:\ProgramFiles\Java\jdk1.6.0_26\bin at the end of the value of Path. Note C: The semicolon in front of \ must be used to separate this path from the previous path. When you enter javac again, the system will search for the directory C:\ProgramFiles\Java\jdk1.6.0_26\bin. You can find javac. I suggest you refer to it, it is written in more detail. For reference, set both JAVA_HOME and classpath.