Under Poisson order-arrival, this paper discusses the economics of job-shop production by a policy approach, and presents an operational basis for job shop control. The model consists of job orders with marginal profits, selection function with backlog, and a production centre with fixed capacity. For the two main policies, PSP and DSP, this model is formulated by semi-Markov decision processes, and the operation rate, mean shop-time and reward rate (including idle cost) are obtained. Finally, the relationship of ‘six order-selection policies is given, and the structure of optimal policies is suggested.