A method for fabricating an interlayer dielectric layer on a semiconductor substrate with a memory cell region and a periphery circuit region is described, wherein semiconductor devices are formed on the memory cell region and the periphery circuit region so as to result in a height variation therebetween. The present method comprises the steps of forming a first dielectric layer blanket-covering the semiconductor substrate, wherein a first height variation exists between the memory cell region and the periphery circuit region. Then, a stop layer is conformally blanket formed on the first dielectric layer. Next, a second dielectric layer is conformally formed on the stop layer. A chemical mechanical polishing process is executed on the second dielectric layer until the stop layer on the memory cell region is exposed. This formation of the structure of a first dielectric layer/stop layer/second dielectric layer is repeated at least two times to achieve a planarized interlayer dielectric layer. A cap layer is formed on the top surface of the planarized interlayer dielectric layer.