A system reliability evaluation method for transmission by a single minimal path in time restriction is disclosed. The single minimal path includes plural arcs between a start node and a terminal node in a flow network. The method includes the steps of providing a virtual network in a computer for simulating the flow network; inputting a transmission requirement, a budget restriction and a time restriction; distributing the transmission requirement in a minimal path of the virtual network for getting a capacity vector to represent the flow distribution of the flow network; under the capacity vector, checking if the transmission cost satisfy the budget restriction for sending the transmission requirement in the minimal path; thereby, defining a lower boundary vector; and computing a system reliability that is a probability of any one capacity vector being lager than or equal to the lower boundary vector.