This paper shows that the junction temperature and input and output power of light-emitting diodes (LEDs) can be determined from forward current directly. Based on the definition of thermal resistance, the relationship between junction temperature and dissipated power of LEDs is analyzed. Electrical power and radiant flux variations with current and temperature are determined experimentally and modelled. These models can be used to simulate the junction temperature in practical applications. As a result, electrical power and optical power can be calculated for each drive current as well. The method is validated by comparing measured and simulated temperature and power values of LED devices mounted on two heat sinks. The same simulation approach can be easily used by diode or application manufacturers to predict solid-state lighting performance.