Summary: The concept of (fuzzy) probability density function of fuzzy random variable is proposed in this paper. Due to the “resolution identity”, we can construct a closed fuzzy number from a family of closed intervals. Using the same technique, we can construct the (fuzzy) probability density function of fuzzy random variable from the known probability density function. The basic idea of the new method is to obtain a method to rank fuzzy number which a fuzzy quantity is related to. The paper must have abstract.