Freak wave may cause danger to ships or people who active at coastal area. Prediction of freak wave can provide early alarm information and hence reduce risks of hazards. However, due to the unclear of freak wave mechanism, deterministic prediction is not possible but stochastic method can be used for research of occurrence probability of freak waves. In this study, nonlinear wave theory is considered for derivation of freak wave occurrence probability. It is function of number of waves and kurtosis of water elevation. Field data from long-term measurements were used for validation which demonstrates the existence of bias. The average root-mean-square-error is 22.4% which is acceptable.