An image processing apparatus which prevents a printing process from taking long. The image processing apparatus is connected to a printing apparatus having a storage unit and transfers image data to the printing apparatus. A size of predetermined image data when a speed at which the predetermined image data is stored in the storage unit and a speed at which the predetermined image data stored in the storage unit is printed are the same is obtained as a threshold size. A size of the image data stored in the storage unit in a compressed form is estimated. A compression method for use in compressing the transferred image data is selected from a plurality of compression methods based on the threshold size and the estimated size of the image data. The image processing apparatus notifies the printing apparatus of the determined compression method.