MICROSCOPE is a fundamental physics space mission which aims at testing the Equivalence Principle (EP) with an accuracy of 10-15. The gravitational signal is measured precisely by a differential electrostatic accelerometer which includes two cylindrical test masses made of different materials. The accelerometer is on-board a drag-free microsatellite which is controlled either Earth pointing or rotating about the normal to the orbital plane with a very stable angular velocity. The expected accuracy of the EP test could be limited by the inaccurate a priori knowledge of the instrument physical parameters associated to the instrument environment on-board the satellite. These parameters are partially measured or estimated by means of ground tests or during the integration of the instrument on the satellite. However, these evaluations are not sufficient and an in-orbit calibration is therefore needed to finely characterize the instrument and to correct the measurements. After the overall presentation of the MICROSCOPE mission and its scientific goal, this paper will focus on the accelerometer and will describe the specific procedures proposed for the inflight instrument calibration.