This paper focuses on an investigation of the proper capillary tube length for an inverter air conditioner. Air to air variable capacity systems with R-22 and R-407C were tested and modeled. First, the optimum refrigerant charge was determined for four capillary tubes at full load condition by varying the mass charge from 1.1 kg to 1.9 kg. The capillary tube lengths were 1.016 m, 0.914 m, 0.813 m and 0.711 m. The two zone model, the distributed model and the combined model were compared to estimate the optimal charge inventory. The combined model analysed a simple path evaporator, a complex path condenser with a two zone model and a distributed model, respectively. It obtained good agreement with experimental results for the system performances and the optimum mass charge. Furthermore, four capillary tubes with specific optimum mass charges were investigated at compressor frequencies in a range of 30-50 Hz. The R-22 capillary tube obtains the best performance with the addition length of 1.016 in at the lowest frequency. Especially, the length of 0.813 in with R-407C is the appropriate size at the operation frequency of 30-35 Hz. The base capillary tube of 0.914 in is optimum at other frequencies. The model prediction agrees with the experimental data in a range of 40-50 Hz. (c) 2006 Elsevier Ltd. All rights reserved.