This study presents a more accurate measurement method for measuring air pressures in the vane segments when a sliding-vane rotary compressor performs suction and compression phases in either stable or unstable rotational speeds. The air pressures in the vane segments when the compressor performs suction and compression phases can evaluate the intake effect of the inlet and seal effect of the vane segment of the compressor, respectively. Two strain gauge type pressure transducers were utilized with an encoder to measure air pressures in the vane segments. The measured air pressure in the vane segment when the compressor performed suction phase increased when the rotational speed increased up to 800 rpm. It decreased when the rotational speed was faster than 800 rpm. When the rotational speed was 1400 rpm, the air pressure in the vane segment reduced to 99.5 kPa compared with the ambient air pressure of 100.3 kPa. Therefore, the dimension or shape of the compressor inlet should be modified to provide the suction air pressure to be closed to the ambient air pressure. When the compressor performed compression phase at the rotational speed of 1400 rpm, the measured air pressure in the vane segment was 153 kPa compared with the designed air pressure of 244 kPa. Therefore, the manufacture precision of the compressor should be improved to reduce the air leakage.