Recently, the accuracy of sound power measurements during the actual operation of a machine in situ improved by using sound-intensity measuring technology. The standard measuring method is being established at ISO. However, one has much difficulty in measuring the sound intensity of large-scale sound sources because the measurement becomes time consuming. On the other hand, there is a simple survey method based on sound-pressure measurements. However, accurate evaluation of the environmental correction factor is then difficult. Based on this background, this paper describes the sound power determination method for large-scale sound sources. Sound intensity measurements were carried out on several large marine diesel engines, which were installed in the ship's engine room or on the manufacturer's test bed where the sound condition was far from free field. Accuracy of the sound power evaluation in such conditions and practicality of the intensity measurement are discussed by detailed analysis of the measured data. Furthermore, a new convenient method called the hybrid method, which is based on the sound-pressure method but needs several points of intensity measurement, is proposed and its practical merit is discussed.