Experimental investigation of the inner diameter in the thermoacoustic cooling system was carried out to confirm the effect for energy conversion efficiency. We carried out an experiment by using the thermoacoustic cooling system with 3 type inner diameters (24 mm, 42 mm, 100 mm). As results, acoustic power and a phase difference between sound pressure and particle velocity was varied. Moreover, it was confirmed that energy conversion efficiency in it changed with the variety of inner diameter. Energy conversion efficiency in it with inner diameter 100 mm was best of all. It is considered that a factor of improvement of energy conversion efficiency was change of phase difference between the sound pressure and the particle velocity.