The Japanese satellite ACSA (Advanced Satellite for Cosmology and Astrophysics) provides us with the first hard X-ray imaging spectroscopic observations on the X-ray sky. The spatial resolved spectroscopy on the shells of supernova remnants revealed non-thermal emission in the hard X-ray bands. Since the hard X-ray spectra are smooth connected to that of the radio synchrotron, the X-rays are also due to synchrotron processes from high energy electrons. Inverse Compton gamma rays are detected from two of the SNRs, hence are established the presence of high energy electrons. Combined with the X-ray, radio and gamma-ray data, we estimated the maximum energy of electrons to be1014&hthinsp;eV.The maximum electron energy is determined from the competing process between the Fermi acceleration and synchrotron energy loss. Protons, of which synchrotron energy loss is negligible, can therefore reach higher energy than electrons, possibly near the knee energy of cosmic rays. ©2001 American Institute of Physics.