A HVMOS transistor structure includes a semiconductor substrate; a gate overlying the semiconductor substrate; a gate dielectric layer between the gate and the semiconductor substrate; a sidewall spacer on each sidewall of the gate; a drain structure in the semiconductor substrate on one side of the gate; an ion well of the first conductivity type in the semiconductor substrate; a source structure in the semiconductor substrate being space apart from the drain structure; and a channel region between the drain structure and the source structure, wherein the channel region substantially consisting of two gate-overlapping regions of the first conductivity type having doping concentrations different from each other.