An experimental study was conducted to investigate the effect of electronic flow control oil the performance of a variable-speed heat pump. A heat pump with two different expansion devices (capillary tube and electronic expansion valve) was tested in a psychrometric calorimeter over a range of outdoor temperatures from -15 to 7C. Heat pump performance was first optimized with respect to charge for each expansion device through cycle-matching tests. Parametric tests also were conducted by changing compressor speed and opening angle for the electronic expansion valve at each outdoor temperature. The refrigeration cycle characteristics of the electronic valve were illustrated using pressure-enthalpy diagrams. Performance enhancement was also analyzed in terms of superheat, heating capacity, and energy efficiency ratio (EER). Comparison of the capillary tube and electronic valve indicated that the superheat significantly improved when using the electronic valve. Also, unit showed larger heating capacity and EER with the electronic valve than with the capillary tube except when the compressor speed was above 95 Hz. Enhancement of heating performance became larger as outdoor temperature decreased.