A numerical analysis for the effect of a cylinder diameter on the state quantities for the irreversible process in a piston-cylinder system was conducted using the LB1S model.  The numerical method is based on the ALE method.  The cylinder-diameter range of D=0.01 m to 0.10 m was chosen.  The effect of the cylinder-diameter on the state quantities was slight in the single compression with the piston velocity of -10 m/s.  The ratio of the average pressure to the pressure of the reversible process at the end of the first cycle ranged from 1.00023 to 1.00462 in the cyclic compression-expansion processes of 20000 rpm.