An image sensor including a substrate, a deep well layer, multiple first sensing units, second sensing units and third sensing units is provided. The first, the second and the third sensing units are located between a first surface and the deep well layer. A ratio between an area of a part of the deep well layer under each first sensing unit and an area of each first sensing unit is a first area ratio. A ratio between an area of a part of the deep well layer under each second sensing unit and an area of each second sensing unit is a second area ratio. A ratio between an area of a part of the deep well layer under each third sensing unit and an area of each third sensing unit is a third area ratio. The first area ratio is greater than the second and the third area ratios.