Maximal fat oxidation (MFO) rate and the exercise intensity that elicits MFO (FATmax-intensity) were designed to evaluate fat metabolism capacity and to provide individuals with a target exercise intensity during prolonged exercise. However, the previous methods of determining FATmax-intensity were time-consuming. The purpose of this study was to examine the validity of FATmax-intensity determined by short-time testing. Nine healthy young men performed ramp exercise, in a short-time test, until exhaustion and 5 constant-load exercises of 60 min each at individual FATmax-intensity determined by ramp protocol (FATmax-intensity(R)), FATmax-intensity(R) ± 5% of peak oxygen uptake (VO₂peak) and FATmax-intensity(R) ± 10%VO₂peak. FATmax-intensity was determined among 5 trials at points of early exercise (10 min) and prolonged exercise (60 min) to evaluate the validity of FATmax-intensity(R). Ten minutes after starting constant-load exercise, FATmax-intensity(R) showed the highest fat oxidation among 5 trials, even though MFO by ramp protocol was overestimated. Therefore, it may be useful for evaluation of fat metabolism to include the measurement of the FATmax-intensity in a routine ramp test. However, because FATmax-intensity(R) did not elicit the highest fat oxidation among 5 trials of 60 min each after starting constant-load exercise, FATmax-intensity(R) may not be effective for prolonged exercise training.