A method of predicting a substrate current in a high voltage device that may accurately predict substrate current components in each of a first region, a second region, and a third region. This may be accomplished by modeling a substrate current component in a third region, in which an inconsistency may occur when a substrate current in a high voltage device is calculated, for example using BSIM3-based modeling. According to embodiments, a substrate current for a third region may be modeled by an expression with a ternary operator, and the modeled substrate current may be added to a substrate current obtained through BSIM3-based modeling.