A method, medium, and apparatus controlling a bit rate in image data encoding. The method for controlling a bit rate in image data encoding includes setting a reference bit amount for encoding a block group composed of a predetermined number of blocks created by dividing image data of a frame, encoding pixel values of the blocks according to a bit depth used to quantize the pixel values of the blocks, checking whether encoding of the frame is completed, and comparing a group bit amount indicating the amount of encoded bits of the block group with the set reference bit amount to adjust the bit depth if encoding of the unit frame is not completed and accordingly repeating the setting of the reference bit amount, encoding pixel values of the blocks, and checking whether encoding of the frame is completed.