A control apparatus configured to control an image forming apparatus in which one of a plurality of fixing devices (8A, 8B) including a standard fixing device configured to perform a fixing operation under a first load and an envelope fixing device configured to perform a fixing operation under a second load which is smaller than the first load is selectively mounted, the image forming apparatus comprising a detecting portion configured to detect density information of a gradation pattern to be formed on a recording material, said control apparatus comprising: an acquiring portion configured to acquire an information corresponding to a kind of the fixing device (8A, 8B) mounted in the image forming apparatus; and an executing portion configured to execute a calibration process configured to calibrate a gradation of an image formed by the image forming apparatus by forming the gradation pattern on the recording material and by detecting its density information with the detecting portion in the image forming apparatus, wherein when the information acquired by said acquiring portion corresponds to the standard fixing device, said executing portion is configured to permit execution of the calibration process, and when the information acquired by said acquiring portion corresponds to the envelope fixing device, said executing portion is configured to prohibit the execution of the calibration process, characterized in that said control apparatus further comprises: a receiving portion configured to receive an input of an execution instruction of the calibration process by a user/operation of the apparatus; and a notifying portion configured to notify prompting of exchange of the fixing device (8A, 8B) to the standard fixing device with receipt of the execution instruction of the calibration process when the information acquired by said acquiring portion corresponds to the envelope fixing device. an acquiring portion configured to acquire an information corresponding to a kind of the fixing device (8A, 8B) mounted in the image forming apparatus; and an executing portion configured to execute a calibration process configured to calibrate a gradation of an image formed by the image forming apparatus by forming the gradation pattern on the recording material and by detecting its density information with the detecting portion in the image forming apparatus, wherein when the information acquired by said acquiring portion corresponds to the standard fixing device, said executing portion is configured to permit execution of the calibration process, and when the information acquired by said acquiring portion corresponds to the envelope fixing device, said executing portion is configured to prohibit the execution of the calibration process, characterized in that said control apparatus further comprises: a receiving portion configured to receive an input of an execution instruction of the calibration process by a user/operation of the apparatus; and a notifying portion configured to notify prompting of exchange of the fixing device (8A, 8B) to the standard fixing device with receipt of the execution instruction of the calibration process when the information acquired by said acquiring portion corresponds to the envelope fixing device. a receiving portion configured to receive an input of an execution instruction of the calibration process by a user/operation of the apparatus; and a notifying portion configured to notify prompting of exchange of the fixing device (8A, 8B) to the standard fixing device with receipt of the execution instruction of the calibration process when the information acquired by said acquiring portion corresponds to the envelope fixing device.