A model for a ground-source heat pump (GSHP) fitted with a variable-speed compressor was developed. A frequency inverter (FI) was used to modulate the GSHP capacity for improving the part-load performance of the system. Year-round dynamic simulations were made using TRNSYS for a general office based on the weather conditions in Hong Kong (HK). Kunming (KM) and Beijing (BJ). Different control schemes for the part-load control were tried and the results compared. It was found that the adoption of a variable-speed part-load control to the GSHP in both the cooling and heating mode operations was better. A reduction in the compressor energy input by minimum 27% could be achieved although a slight increase in the borefield fluid circulating pump energy consumption was inevitable. The peak borefield fluid temperatures were also reduced which allowed the borehole lengths to be shortened by at least 4% and the initial cost lowered accordingly. An economic analysis indicated that with the reduced borehole lengths, the payback periods were no longer than 0.4 months. This highlighted the merit of employing a variable-speed part-load in a GSHP system as the initial cost might also be saved besides the running costs under a wide range of climatic conditions. (C) 2010 Elsevier Ltd. All rights reserved.