The exothermic nonradiative relaxation processes from photoexcited nitrobenzene (NB) are studied by the picosecond time-resolved transient grating method. The decay rate constants and energy of the excited NE are determined. The lifetime of the lowest excited singlet state is found to be very short (less than or equal to 10 ps) and a surprisingly short lifetime of the lowest excited triplet state (similar to 480 ps) is detected. From quantitative measurements of the thermal energies released from the excited states of NE, the lowest excited triplet state is determined to be located at (22-27) x 10(3) cm(-1). The triplet formation is very efficient and its quantum yield is found to be grater than 0.80. From the temperature and solvent dependence of the triplet lifetime, it is suggested that motions of the nitro group may cause the surprisingly rapid relaxation from the lowest excited triplet state to the ground state. The photophysical properties and excited states of monosubstituted nitrobenzenes are also discussed.