An image forming system includes a first image forming apparatus, a second image forming apparatus disposed upstream in a sheet conveying direction and connected to the first image forming apparatus, and a post-processing apparatus that receives a sheet at a speed depending on a conveyance speed of a sheet discharged from the first image forming apparatus and that performs post-processing on the sheet. When a print job including a simplex image forming mode and a duplex image forming mode is to be executed, in the simplex image forming mode, a sheet is discharged in a reversed state to the post-processing apparatus. In the duplex image forming mode, the second image forming apparatus forms an image on a back side of a sheet, the first image forming apparatus forms an image on a front side of the sheet, and the sheet is discharged in a reversed state to the post-processing apparatus.