Image sensor having test pattern and offset correction method thereof

An image sensor includes a substrate including an active pixel and a test pattern, wherein the test pattern is located adjacent to the active pixel, wherein the active pixel comprises a first photodiode, a floating diffusion, a first channel provided between the first photodiode and the floating diffusion, and a first transfer gate electrode provided over the first channel, wherein the test pattern comprises a first test photodiode, a test floating diffusion, a second channel provided between the first test photodiode and the test floating diffusion, a first test transfer gate electrode provided over the second channel, and a first contact plug connected to the first test photodiode, and wherein the first test photodiode, the test floating diffusion, the second channel, and the first test transfer gate have substantially the same alignment errors as the first photodiode, the floating diffusion, the first channel, and the first transfer gate electrode, respectively.