An image forming apparatus capable of efficiently performing an image forming processing even in a case where a post-processing apparatus performs a post-processing during a both-sides printing. A first time period needed by a first both-sides image forming processing and a second time period needed by a second both-sides image forming processing are computed in a case where a post-processing unit performs a post-processing on a recording sheet formed images on both sides thereof. The first time period and the second time period is compared, and any one of the first both-sides image forming processing and the second both-sides image forming processing is selected based on the comparison.