In this paper the theoretical cycle of OHV type single cylinder engine which carburetes hydrogen and methanol is compared with its actual cycle.That the change in number of molecules in combustion is independent of the excess air ratio and the residual gas fraction. When hydrogen methanol feed ratio is equal to or less than 0.0625, the number of molecules in combustion is increased and it usually has beneficial effect on engine performance, which is not the case when hydrogen only is supplied.Final temperature (or pressure) of compression and theoretical thermal efficiency increase progressively as hydrogen methanol ratio and excess air ratio increase.The heating value and final temperature of combustion decrease gradually in proportion to the increase in excess air ratio and to the decrease in hydrogen methanol ratio.However, when hydrogen methanol ratio is decreased rise of the final pressure of combustion is seen caused by the mole variation with combustion.The temperature rise ratio for combustion is nearly independent of hydrogen methanol ratio, whereas at a relatively high excess air ratio, only small curve differences between hydrogen methanol ratio are found. In contrast, the effect of hydrogen methanol ratio on degree of explosion is larger at lower excess air ratio.It is observed that the variation in actual engine brake horsepower by the increase in amount of hydrogen supplemented to methanol is almost the same, when it is compared with the results of the calculation method which was treated in this work.