To enable a large amount of printing jobs to be efficiently carried out with low running costs, thereby preventing inconveniences such as complicated operations required of an operator, generating desired data for the operator, and allowing the operator to work more efficiently, an image processing system comprising a plurality of image forming apparatuses is provided, control is provided such that a group of image data to be printed are separated into a plurality of groups and transmitting the groups to the plurality of image forming apparatuses, image processes are executed on the image data to be transmitted depending on printing attributes of the image forming apparatuses to which the image data are to be transmitted, in a manner such that different processes are executed for respective ones of the groups and the image data that have been processed for the respective ones of the groups are transmitted to respective corresponding ones of the image forming apparatuses. Further, a first image forming apparatus is controlled to form images on sheets with image data of a group of image data which are for pages corresponding to image data having a first attribute, the group of image data including the image data having the first attribute, and image data having a second attribute, and a second image forming apparatus is controlled to form images on sheets with image data for pages corresponding to the image data having the second attribute. The first image forming apparatus is controlled to operate by selecting one of a plurality of operation modes depending on a command from a user, the operation modes including an insertion mode in which image data of the group of image data which are for the pages corresponding to the image data having the first attribute are formed on sheets from a first feeding unit while sheets from another feeding unit which is different from the first feeding unit are inserted into pages at positions corresponding to image data of the group of image data which have the second attribute, and a non-insertion mode in which image data of the group of image data which are for the pages corresponding to the image data having the first attribute are formed on sheets from the first feeding unit while the control means inhibits insertion of sheets from the another feeding unit which is different from the first feeding unit into the pages at the positions corresponding to the image data of the group of image data which have the second attribute.