A method of defining poly-silicon growth direction includes Step 1, forming a buffer layer on a substrate; Step 2, forming a regular graphene array; Step 3, forming an amorphous silicon thin film on the buffer layer, which the regular graphene array has formed thereon; and Step 4, transferring the amorphous silicon thin film into poly-silicon with an excimer laser anneal process. The growth direction of the poly-silicon as being formed can be controlled according to the present method of defining poly-silicon growth direction. Accordingly, the grain size of the poly-silicon can be raised.