Abstract When the air conditioning and heating systems operate in low speed regions, the efficiency of inverter compressor is relatively low, owing to the reduction in motor efficiency. Nevertheless, the annual operating time of compressor is largely occupied by low speed regions. Hence, experimental and numerical analyses were conducted to improve compressor efficiency in these regions, and various pressure conditions were considered. The significant difference in the volumetric efficiency with compression ratios was observed through the experiments. To reduce the decrease in volumetric efficiency at high compression ratio, a geometric combination of the compression part was considered through a numerical analysis. The numerical results showed that the volumetric efficiency at small cylinder slenderness ratio was relatively high and increased by about 6.3% at a compression ratio of 7. Then the cooling capacity increased by 8.77%, while the input work showed a relatively small increase of 2.44%.