A method was developed to determine the petroleum coke particle size in illuminant flame by a laser-Doppler velocimeter. The particle size was determined by taking the wave number into account in addition to the pedestal amplitude. The petroleum coke particles were blown into the vertical test pipe from the lower part of the pipe and were heated to 850° C by electricity. The measurement points were at the 50 mm, 450 mm and 900 mm up the pipe entrance. The measurement result shows that there was obviously particle size enlargement at the process of the burning, and there was not obviously crash phenomena at the process.