A high voltage transistor structure including a substrate, a first drift region, a second drift region, a first cap layer, a second cap layer, a gate structure, a first source and drain region, and a second source and drain region is provided. The first and second drift regions are disposed in the substrate. The first and second cap layers are respectively disposed on the first and second drift regions. The gate structure is disposed on the substrate and located over at least a portion of the first drift region and at least a portion of the second drift region. The first and second source and drain regions are respectively disposed in the first and second drift regions and located on two sides of the gate structure. The size of the first drift region and the size of the second drift region are asymmetric.