A high voltage semiconductor device is provided. A first-polarity buried layer is formed in the substrate. A first high voltage second-polarity well region is located over the first-polarity buried layer. A second-polarity base region is disposed within the first high voltage second-polarity well region. A source region is disposed within the second-polarity base region. A high voltage deep first-polarity well region is located over the first-polarity buried layer and closely around the first high voltage second-polarity well region. A first-polarity drift region is disposed within the high voltage deep first-polarity well region. A gate structure is disposed over the substrate. A second high voltage second-polarity well region is located over the first-polarity buried layer and closely around the high voltage deep first-polarity well region. A deep first-polarity well region is located over the first-polarity buried layer and closely around the second high voltage second-polarity well region.