This paper theoretically investigates the effects of the temperature and flowrate of cold seawater on the net output of an OTEC plant. Parameters of pipe length, pipe diameter, seawater depth, and the flowrate of seawater are considered. It shows that a maximum output of the net work exists at a certain flowrate of cooling seawater. The output work is higher for a larger ratio of warm to cold seawater flow rate. For a lower cold seawater temperature, the maximum net output and the corresponding required flow rate of cold seawater becomes larger. The pipe diameters corresponding to maximum net output increase with decreasing cold seawater temperature for fixed flow velocity in pipes. In addition, a larger maximum net output can be obtained by employing a higher temperature of the surface warm seawater.