An output compensation circuit of an image sensor includes a first current mirror circuit, a first current generator circuit and a second current generator circuit. The first current mirror circuit, coupled to a select transistor of a pixel circuit of the image sensor, is configured to, in response to a first current, generate a second current flowing through the select transistor. The select transistor is selectively turned on according to a power supply voltage. When the select transistor is turned on, the pixel circuit outputs the second current through the select transistor. The first current generator circuit outputs a compensation current, serving as a first portion of the first current, to the first current mirror circuit in response to a variation in the power supply voltage. The second current generator circuit outputs a reference current, serving as a second portion of the first current, to the first current mirror circuit.