We model the underlying asset with SABR model. According to the Black-Scholes formula, the implied volatility under the SABR model has a closed-form formula. Then we consider about the estimation as the volatility is observable or not. When the volatility is unobservable, the volatility state is replaced by proxies based on the implied volatility. Finally we obtain the estimator by applying the maximum likelihood estimator, and use the Monte Carlo simulation to complete the research.