A MOS P-N junction diode device includes a substrate having a first conductivity type, a field oxide structure defining a trench structure, a gate structure formed in the trench structure and a doped region having a second conductivity type adjacent to the gate structure in the substrate. The method for manufacturing such diode device includes several ion-implanting steps. After the gate structure is formed by isotropic etching using a patterned photo-resist layer as a mask, an ion-implanting step is performed using the patterned photo-resist layer as a mask to form a deeper doped sub-region. Then, another ion-implanting step is performed using the gate structure as a mask to form a shallower doped sub-region between the gate structure and the deeper doped sub-region. The formed MOS P-N junction diode device has low forward voltage drop, low reverse leakage current, fast reverse recovery time and high reverse voltage tolerance.