The research on heat transfer models of geothermal ground heat exchangers (GHEs) of ground-coupled heat pump (GCHP) system has recently advanced greatly. However, although it is important to optimize the design size of GHEs for reducing the total length of GHEs, the optimization of GHEs by means of models is a little. This paper describes the interior simulation models of borehole in which single U-tube and double U-tubes are each buried. The analytical solutions concerning the borehole's exterior heat transfer are given. All the factors that exert influences on the design size of GHEs are described based on the results of the heat transfer models. These significant parameters consist of the centre to centre distance of the U-tube, thermal conductivity of the backfill material, distance of adjacent boreholes, types of circulating liquid or underground medium, arrangement of boreholes, and the minimum temperature of the circulating liquid which enters the heat pump. Using the simulation models and computer programming, the influence degrees of the above factors are discussed in terms of the adoption of different values or types. Therefore, the initial cost and the economic performance of the system are respectively dropped and improved. The investigation on optimization of GHEs is favourable for the further development of GCHP technology. (C) 2015 Elsevier Ltd. All rights reserved.