The thermodynamic effects on the suction performance of a liquid oxygen (LOX) pump, observed during liquid rocket engine combustion tests, were investigated. Owing to the pump rotational speed and inlet pressure variation in the short duration of the engine starting, LOX pump head drops occurred occasionally depending on the initial pump inlet condition. Because the engine tests were performed at various inlet temperature and pressure settings of the LOX pump, the resulting suction performance behaviors of the LOX pump were varied. The critical cavitation number, at which the pump head drops 3%, was considered as the main parameter for representing the pump suction performance. The suction performance behaviors shown in the engine tests were investigated based on the classical theory of thermodynamic effects on cavitation. The LOX pump component test results in water, the LOX pump assembly suction performance test results in liquid nitrogen, and the J-2 LOX pump test results available in the open literature were used in the analysis and comparison. It was found that the critical cavitation number ratio of a pump could be expressed as a function of the thermodynamic parameter Σ*. For a given LOX pump flow condition, Stepanoff’s B-factor was almost constant at the specified head drop condition; as a result, Brennen’s time scale ratio βwas not constant but varying with Σ*. The argument of the geometrical similarity of pump cavitation at the critical condition of the pump head drop was addressed in conjunction with the constancy of Stepanoff B-factor.