Coalbed methane (CBM) is an important global energy resource and liquefaction is suggested to best utilize it. Different from ordinary natural gas, CBM usually contains a high proportion of nitrogen, which cannot be removed by purification procedures applied in ordinary natural gas liquefaction processes. One approach for separating nitrogen from CBM is by distillation after liquefaction. In this way, nitrogen is liquefied together with methane, and the liquefaction system performance may change along with the nitrogen content of CBM feed gas. The liquefaction process adopting nitrogen expansion with propane pre-cooling is usually considered suitable for small-scale liquefaction plants due to its simplicity and is the focus of this paper. Taking the unit product liquefaction power consumption as the major index for analysis, optimum parameters of the liquefaction process for CBM feed gas containing different nitrogen contents are calculated. Based on the optimization results, the effects of nitrogen content as well as the other two important technical indexes (liquefaction rate and methane recovery rate) on system performance are also investigated. (C) 2009 Elsevier Ltd. All rights reserved.