Abstract. The next NASA Small Explorer (SMEX) mission, IXPE ( Imaging X-ray Polarimetry Explorer) , will be launched in 2021, with three identical X-ray mirror module assemblies and Gas Pixel Detectors (GPD). IXPE introduces the capability for X-ray polarimetric imaging, uniquely enabling the measurement of X-ray polarisation with scientifically meaningful spatial, spectral, and temporal resolution. During its baseline two-year operation, IXPE will have polarisation measurements on a few dozen sources, including various types of neutron stars, blackhole systems, active galactic nuclei, and supernova remnants, help to probe the origin and destiny of our universe. A series of simulation activities together with the calibration for flight units are in progress. The polarimeter design and expected performance will be discussed.