A control circuit for compensating output loss of a power converter includes a sampling voltage generator, a time-to-voltage converter, and a compensation signal generator. The sampling voltage generator generates a sampling voltage corresponding to a detection voltage according to a first reference current, a second reference current, and the detection voltage. The time-to-voltage converter generates a corresponding voltage according to a period of a gate control signal controlling a power switch of a primary side of the power converter and a discharge time of a secondary side of the power converter. The compensation signal generator generates a compensation signal compensating the output loss according to the sampling voltage and the corresponding voltage.