The paper describes a study of procedures on risk assessment of liquefied gas tankers. The study consists of : a) a proposal of a simple analysis method for the assessment, b) assumption of essential failures and human errors, and their probability involved, and c) an example of the assessment applied the proposal to a 70, 000 m³ Ref. LPG tanker. Concludedly, following formula is proposed for safe design of liquefied gas tankers; P_i ≤ A . C_i ^{- n} or C_i ≤ ( Pi / A ) ^{-1/ n} P_i is probability of accident (i) per year. C_i is coefficient of accident effect and takes 100 for the maximum accident assumed, such as collision in a harbour, and reasonably decreases by risk level in each accident. A is 5 × 10^-5 to 10^-6 and n is 0.7 for Ref. LPG, LNG, Ethylene tankers.