The heat-releasing processes of the photoexcited states of 9,10-diazaphenanthrene (DAP) in ethanol have been directly detected using the picosecond time-resolved transient grating (TG) method. From the heat-releasing rates, a surprisingly short lifetime of the lowest excited triplet state-(similar to 13 ns) and a relatively long lifetime of the lowest excited singlet state (4.3 ns) are detected. By measuring the heat energy coming from the excited state of DAP together with the fluorescence quantum yield measurement, the quantum yield of the triplet formation is found to be about unity. The unique photophysical properties of DAP are also discussed.