Measured energy use of buildings demonstrated large discrepancies even between buildings with same function and located in similar climates. Among various factors contributing to the discrepancies, occupant behavior is a driving factor. Occupant behavior is also one of the most significant sources of uncertainty in the prediction of building energy use by simulation programs. How occupants set the comfort criteria (including thermal, visual, and acoustic), interact with building energy and services systems, and response to environmental discomfort directly affect the operation of buildings and thus their energy use. This study employs building simulations to evaluate the impact of occupant behavior on energy use of private offices with single occupancy. Typical occupant behavior we studied includes how an occupant sets comfort criteria, operates lights, office equipment, space thermostat, and HVAC systems. The behaviour is categorized into three workstyles: 1) austerity–occupants are proactive in saving energy, 2) standard–average occupants, and 3) wasteful–occupants do not care about energy use. The simulation results demonstrate the impact of occupant behavior on building energy use is significant, and even so at the energy end use levels such as lighting, space cooling and heating. For atypical single-occupancy office room, compared to the standard or reference work style, the austerity work style consumes up to 50% less energy, while the wasteful work style consumes up to 90% more energy. Three methods are proposed to model occupant behavior depending upon the complexity: 1) use EnergyPlus directly, 2) use the advanced feature of EnergyPlus-Energy Management System, and 3) use modified code of EnergyPlus. Our study provides a method to evaluate energy impact of occupant behavior, which can be a good tool for decision makers of behavioral programs that target energy savings in buildings.