The transient process in a full-scale municipal solid waste incinerator was investigated to assess the effect of the grate movement and waste Feeding cycles. Inside-bed measurements of temperature and O-2 were carried out and an 18-hour record of furnace temperature, airflow. and fine gas compositions was analyzed using discrete Fourier transform. The transient burning process was also simulated numerically by solving the governing equations for both the solid and gas phases. It is found that fluctuations in the furnace temperature and flue gas O-2 level correspond to the waste feeding and state movement cycles. either in phase or out of phase (depending on the length of the cycles), while combustion inside the bed is dominated by a series of big spikes and dips in both local temperature and O-2 level Theoretical calculations indicate that each grate movement causes a surge (by 35%) in the volatile release rate but a depression (by 24%) in the char burning raw. The overall bed-burning rate fluctuates between 90 and 110% of its average.