An image forming apparatus and an image transferring method therefor are provided. The apparatus includes a plurality of photosensitive units, an intermediate transfer member, and a sheet-conveying unit. Each of the photosensitive units bears a first toner image. The intermediate transfer member is in contact with the photosensitive units, onto which the first toner images are transferred from the photosensitive units and superimposed to form a second toner image. The intermediate transfer member is rotationally driven at a first speed. The sheet-conveying unit is used for transporting a sheet past the intermediate transfer member at a second speed different from the first speed, so that the second toner image on the intermediate transfer member is transferred onto the sheet.