The purpose of this study is to grasp the actual variation in the inflation pressure of large-sized, off-the-road pneumatic tires. So far the inflation pressure of a tire was regarded as in constant under operation. In this research, the variation of inflation pressure was investigated in detail for a period of four hundred days. The object of the investigation is 3300-R51 off- the-road tires having an initial inflation pressure of 850 kPa used in the heavy dump truck working in a quarry site of limestone at Kochi Prefecture of Japan. The average variation of the inflation pressure during 400 days and 9 pressure control per day was to be controlled at 21 kPa.The variation of 52 kPa in the inflation pressure within 8 hours per ordinary working day was also measured and was about 2.5 times the controlled variation. The temperature near the tire carcass was also measured. It was observed that the variation of the inflation pressure is accompanied by the variation of the temperature near the tire carcass. Furthermore, it was verified that the variation in the inflation pressure causes changes of load capacity, profile, contact pressure distribution and form of wear. As a result, the air pressure fluctuation could be controlled by using heat insulator between air layer and carcass.In addition to the aspect of conventional inflation pressure control, the research also involves various aspects of tire performances.