module import error, what's wrong?

[smith jack]

from common.URLTool import URLTool
tool = URLTool()

Traceback (most recent call last):
  File "E:\workspace\url\test.py", line 7, in ?
    from common.URLTool import URLTool
ImportError: No module named common.URLTool

URLTools is a class write by myself, it works well ,but cannot be
imported in the other package, what's wrong?