Factors determining thermal efficiency of a spark-ignition engine fueled with ammonia/hydrogen mixture were investigated. A diesel based four-stroke single-cylinder spark-ignition engine with a stroke volume of 412 cm³ was used. Different compression ratios and two pistons with different squish areas were used. Total equivalence ratio was kept at unity and mixing ratio of ammonia and hydrogen were changed. It is shown that higher compression ratio and larger squish velocity give wider stable operation range of the engine. For high compression ratio with high squish piston, operation with pure ammonia was possible. The indicated thermal efficiency was expressed as a product of four factors; (i) the effect of compression ratio and specific heat ratio, (ii) the effect of change in molar number, (iii) the effect of degree of constant volume, and (iv) the combustion efficiency. Also, the indicated mean effective pressure was expressed as a product of the aforementioned four factors and (v) the input heat. It is shown that the model expresses the actual indicated thermal efficiency and the actual indicated mean effective pressure quantitatively to some extent and that the expansion of the stable operation range can be attributed to the improvement of the combustion efficiency.