This paper deals with the N policy M/G/1 queue with working breakdowns. The supplementary variable and probability generating function techniques are implemented to develop the steady-state results. The stability condition of a stable queue, as well as several system performance measures, are also derived. A two-stage optimization method is employed to determine the optimal threshold N and the optimal joint values of two mean service rates until the stability constraint is satisfied. To demonstrate the effectiveness of two-stage optimization method, some numerical results are presented. Finally, we carry out sensitivity analysis for the expected cost function with numerical illustrations.Symmetry 2020, 12, 583 2 of 16 of maximum entropy to investigate the N-policy M/G/1 queue with server breakdowns and general startup times. Using the same approach, Ke and Lin [10] approximated the steady-state probability distributions of the queue length for the N-policy M [x] /G/1 queue with an unreliable server and a single vacation. The optimal control of the N-policy M/G/1 queueing system with server breakdowns and general startup times was examined by Wang et al. [11]. They applied the direct search method to determine the optimal threshold N at the minimum cost. Jain and Bhargava [12] performed cost analysis of the N-policy for the machine repair problem with mixed standbys and an unreliable server. An N-policy M X /M/1 queueing system with server startup and breakdowns was analyzed by Vemuri et al. [13], where service was in two phases. Singh et al. [14] focused on the investigation of the N-policy queue with an unreliable server, state-dependent arrival rates, two phases of service, and m phases of repair. Moreover, Yang and Ke [15] applied the supplementary variable technique to analyze the (p, N)-policy M/G/1 queue with an unreliable server and a single vacation. Chen and Wang [16] address the sensitivity analyses of a retrial machine repair problem with warm standby units and a single server under the N-policy.Over the years, there has been extensive literature on N-policy queues with server breakdowns, in which the server stops working completely during the breakdown period. However, there are no studies investigating the N-policy queue with working breakdowns. This queueing model accommodates many real-world systems, such as computer systems, assembly systems, and manufacturing systems. Thus, it motivates us to focus on the analysis of the N-policy M/G/1 queue with working breakdowns. The purpose of this paper is three-fold.