Provided is a hydroelectric power generator for a pipeline that is installed at a position in a pipeline to generate electricity using a rotating turbine by inducing a flow velocity of water therein and uses environment-friendly energy through the water velocity while varying in size in accordance with the size of a pipeline to generate electricity even at a low-speed stream. The hydroelectric power generator for a pipeline generates electricity with high efficiency by increasing an angular speed at the same flow speed by adjusting the number and radius of turbine blades.