A fabricating method of transistors includes providing a substrate with numerous transistors thereon. Each of the transistors includes a gate structure. A gap is disposed between gate structures adjacent to each other. Later, a protective layer and a first dielectric layer are formed in sequence to cover the substrate and the transistors and to fill in the gap. Next, numerous buffering particles are formed to contact the first dielectric layer. The buffering particles do not contact each other. Subsequently, a second dielectric layer is formed to cover the buffering particles. After that, a first planarization process is performed to remove part of the first dielectric layer, part of the second dielectric layer and buffering particles by taking the protective layer as a stop layer, wherein a removing rate of the second dielectric layer is greater than a removing rate of the buffering particles during the first planarization process.