The sustainability of transportation system can be evaluated by three indicators: Environmental Sustainability, Social Sustainability, as well as Economic and Financial Sustainability. Compared to private motorized transportation, mass rapid transit (MRT) has absolutely better performance on Environmental Sustainability and Social Sustainability. However, the capital cost of MRT is significantly higher than other public transportation systems. The construction cost of Taipei Mass Rapid Transit (TMRT) amounted to 2.8 billion dollars per kilometer. The huge capital investment may cause to the financial problems of the operator and government. For the aspect of operating, the self-liquidating capital cost of the second stage of TMRT will become the financial pressure on Taipei Rapid Transit Company (TRTC). Therefore, in order to reach the Economic and Financial Sustainability, the operating performance and the optimal network scale of MRT should be evaluated seriously. This study aims to analyze cost characteristics and minimum efficient scale (MES) of TMRT based Translog cost function. The data used in this study is obtained from TRTC for 1996-2006 seasonal operating data. By the estimated Translog cost function, one can analyze the economic characteristics of TMRT. The result indicates that TMRT has strong characteristic of economies of density, and the elasticity of substitution is not significant. For the MES analysis, it is shown that MES occurs when the network length is 140 km and the average ridership is 2.36 million trips per day. This implies that the government should evaluate the necessity of the expanded routes of TMRT. To reach the Economic and Financial Sustainability, other public transportation system, such as light rail and bus rapid tansit, could be considered as another option for some expanded MRT routes with lower demand.