In this paper, we will describe two methods as we called Poisson method and Generic Expectation Function (GEF) method for using to find pipe failure probabilities of water distribution systems which is implicitly design by engineers. In reliability, one is concerned with system failure. In order to develop GEF method using means and coefficients of variation of input random variables through employing several probability distributions, normal and lognormal distributions are adopted. In this paper, 10 water distribution systems which are located in Terengganu, Malaysia have been used for illustrating the mentioned above methods from which the comparison can be discussed. Besides that, hydraulic simulation software, EPANET has been applied to get the input variables for each project. Failure probability of each pipe is focus on failure probability of pipe to fulfill the demand P(A) also pipe replacement probability P(B).