Boron transient enhanced diffusion (TED) causes reverse short channel effect, and threshold voltage (V_t) increases abnormally, which seriously affects the device performance and yield. The V_t increment is different with different channel size. To investigate channel impurity distribution and the influence of channel size on TED, V_t adjustment ion-implantation, LDD ion-implantation and carbon co-implantation experiments were performed on 40 nm CMOS process, measuring V_t with different channel size and process, and using TCAD tools for process simulation. Experiments results show that V_t rises first with channel length decreasing and then drops sharply after reaching the maximum at 0.55 μm. The V_t -L curve slope decreases when channel width decreases. V_t decreases with channel width and it is decreasing faster and faster when channel length is constant. The interstitial Si atoms are injected by LDD ion-implantation and diffuse into channel, while boron pile up at the LDD-channel boundary. However, boron leak into the shallow trench isolation (STI) area at the corner formed by LDD and channel. Both pile-up and leakage control boron concentration distribution in channel. The V_t shift induced by boron TED is channel size-dependent, and high energy carbon co-implantation and infrared rapid thermal anneal can suppress TED effect effectively.