Exhaust gas emissions and piston wall temperatures under decelerating operation when fuel amounts abruptly decreased in a small DI diesel engine were investigated and the results were compared with those in steady state and accelerating operation. Whole exhaust gas emitted during two consecutive cycles was sampled from the start of deceleration to about 3000 cycles after the start of deceleration using a newly developed gas sampling system. The results indicated that all kinds of emissions examined showed transient behavior after the start of deceleration before reaching a steady state condition corresponding to the change in the piston wall temperature, a simple exponential function of the cycle number. In particular, the THC emission during the deceleration increased gradually with decreasing the piston wall temperature before reaching a steady state concentration. The THC emissions at both acceleration and deceleration could be predicted by the concentrations at the steady state operation with the same fuel amount and piston wall temperature except immediately after the start of acceleration. The NOx emission increased with a decrease in the piston wall temperature during decelerating operation in the same manner as in steady state operation.