In this paper, we show that the Newton-Raphson method is suitable to locate the optimal ordering time for the inventory model taking account of time value. First, we prove that the sequence constructed from Silver-Meal heuristic will converge. Second, we can take a point near to the limit point as the starting point for the Newton-Raphson method. Finally, from the numerical examples, we know that the Newton-Raphson method is superior to the bisection algorithms that are cited by two recent papers.