Pool boiling tests were conducted with plain steel tubing using pure ammonia and a mixture of ammonia and a miscible lubricant. By comparing the results, the effect of miscible lubricant on shell-side boiling heat transfer coefficient of ammonia was quantified. The experiment covered saturation temperature from -23.3°C to 7.2°C (-10°F to 45°F) and heat flux up to 60 kW/m2 [19,000 Btu/(h·ft2)]. Nucleate boiling of ammonia took place outside a plain steel tube of 19.1 mm (0. 75 in.) diameter. The tube was heated by liquid flowing inside to simulate a real heat exchange device. A polyalkylene glycol lubricant, miscible with ammonia, was tested at 0%, 1%, 5%, and 10% mass concentrations. The experimental results showed that under a particular saturation temperature and heat flux, the heat transfer coefficient generally first decreased with an increase in oil concentration up to 5%. It was then followed by an insignificant increase in the coefficient with a further increase in oil concentration to 10%. At a low temperature and a high heat flux, the heat transfer coefficient decreased with the lubricant concentration and stayed constant. The effect of miscible lubricant strongly depends on saturation temperature and weakly depends on heat flux. The largest degradation of heat transfer performance due to the miscible lubricant occurs at the present low temperature of -23.3°C where the heat transfer coefficient can be reduced by up to 33%. A general correlation was developed for heat transfer coefficients both with and without lubricant effect.