This paper presents an EMQ model which determines an economic manufacturing quantity in an unreliable machine. It is assumed that life time of a machine is generally distributed and it requires an instantaneous repair time when a machine fails. Depending on various types of failure rate function of a machine, it is discussed how to determine an EMQ and prove its uniqueness. Variations of an EMQ depending on repair cost are also examined. Through numerical experiments, extensive investigations are carried out on the effects of repair cost and setup cost to an EMQ as well as average cost, and some interesting behaviors are observed.