To solve the problems in design speed method for calculation of horizontal curve radius in the current specification, reliability design method is presented. Based on Unified Standard for Reliability Design of Highway Engineering Structures, the reliability of curve radius was defined. With lateral slide of vehicle traveling around horizontal curve as constraints, the performance function of curve radius was established and models for calculation of its reliability and reliable indicator were deduced. For solution of the models, analytical methods and Monte Carlo method were recommended. With selfdeveloped program, a case study was conducted to illustrate the reliability design method in calculating curve radius of an accidentprone highway. Based on psychological investigation, target reliability, reliable indicator, and minimum curve radiuses were recommended for design of curve radius of various grades of twolane highway. Hidden reliability of the current specification was calculated. The case study and the hidden reliability analysis together shows that even if the design result with design speed method satisfy requirement of the specification, it maybe not reach the standard of reliability design method. Therefore, Reliability design method for horizonal curve radius of highway alignment is helpful for safety improvement.