In this paper we show an approximation for $\hat f(u)$, where u is a fuzzy interval and $\hat f$ is obtained from a continuous function f applying the Zadeh's extension principle. In this process, we used a decomposition for u and the linear spline function for the approximation of f. A bound for the error in the approximation is given. Also, some examples are presented. © 2009 World Scientific Publishing Company.