A method for forming flash memory units is provided. After a logic gate in a select gate PMOS transistor area is separated from a logic gate in a control gate PMOS transistor area, P-type impurities implanted into the logic gate in the select gate PMOS transistor area are diffused into an N-type floating gate polysilicon layer to convert the N-type floating gate into a P-type floating gate by a subsequent high temperature heating process, so that it is possible to successfully form a select gate PMOS transistor having a small surface channel threshold value in a 55 nm process flash memory unit, and achieve mass production. Further, a two-step growth process of the logic gate and a process for separating the logic gate can form a surface channel of the select gate PMOS transistor having a smaller threshold value without affecting the floating gate doping of the control gate PMOS transistor.