The main idea of this paper is inspired by a paper written together by my advisor Dr. Hang, Dr. Huang of the Industrial Technology Research Institute, and Dr. Chao of Institute of Applied Mechanics of National Taiwan University[quant-ph/0506153 v1,2005]. After some mathematical calculations we can extend the WKB approximation to treat position-dependent effective mass problem (PDEM). Then we did simulation on a model PDEM problem to compare the well-know closed form solution and the extended WKB approximation. We demonstrated that the extended WKB approximation not only can obtain the eigenvalues very accurately, but also is very useful to estimate the distribution of the wave function. We also found the modulation on the oscillations of wave function under PDEM by the extended WKB approximation.