Khlui-Phiang-Aw is a reedless wind instrument and its structure is similar to a long pipe. Blowing Khlui-Phiang-Aw forms a column of air jet to hit a wind cutter, which is a sharp edge in front of the jet outlet. This is the input energy to the instrument. The jet is then altered by the wind cutter, and the alternated pressure is then radiated as sound. This mechanism produces oscillation of the air flow. Most energy is lost due to friction at the wall. The objective of this work is to study the flow in Khlui-Phiang-Aw of which cannot be observed with the naked eyes and can only be perceived by a computer simulation. The simulation is performed by implementing a computational fluid dynamic model and utilize a PC to solve partial differential equation. The result shows that blowing Khlui-Phiang-Aw leads to the accumulation of pressure inside the pipe causing pressure gradient between the inside and outside of the instrument. When the pressure gradient exceeds a threshold, the air at the wind cutter flows out. Thus the pressure inside the pipe is reduced until it is lower than a threshold; consequently, the air at the wind cutter flow into the pipe and this process repeats itself from the beginning again. The simulation shows the correlation between the air velocity that swings up and down at the wind cutter and the pressure gradient around the wind cutter.