A method for a programmable primitive setup in a 3D graphics pipeline is introduced to contain at least the following steps. Information about first and third primitives is obtained from a buffer. The information about all or a portion of the first primitives is packed and sent to an SS (Setup Shader) thread. Information about a second primitive to be clipped is packed and sent to a GBS (Guard-Band-clipping Shader) thread. The information about all or a portion of the third primitives is packed and sent to an AS (Attribute Shader) thread.