The Legislative Council in Hong Kong has approved a funding of USD$8.60 billion to build the high-speed rail (HSR) line linking mainland China. HSR is a break-through technology that allows trains running at a speed over 250 km per hour. The most controversial part of the HSR investment is whether its cost could be compensated by the social benefits. In this study, a cost-benefit analysis of the Hong Kong to mainland HSR (HKM-HSR) line is carried out. First, all the direct and indirect costs, and social benefits are defined; then, monetary equivalents are assigned to these elements; third, all the future values are discounted into present values and aggregated. The results show that the project has a positive net present value (NPV) up to USD$2,068.49 million, which proves that the investment is worth. In addition, other transport alternatives, i.e. the existing roadway and conventional railway, are examined and compared with HKMHSR, which unveils that HSR has the largest positive NPV among these three passenger transportation modes because of its excellent performance in ticket revenue, travel time savings and safety improvement.