A power transistor chip with built-in enhancement mode metal oxide semiconductor field effect transistor and application circuit thereof provides an enhancement mode metal oxide semiconductor field effect transistor in association with two series connected resistors to act as a start-up circuit for the AC/DC voltage converter. The start-up circuit can be shut off after the pulse width modulation circuit of the AC/DC voltage converter circuit works normally and still capable of offering a function of brown out detection for the pulse width modulation circuit as well. Besides, the enhancement mode metal oxide semiconductor field effect transistor is built in the power transistor chip without additional masks and processes during the power transistor chip being fabricated such that the entire manufacturing process is simplified substantively with the economical production cost.