A model of synchrotron emission from supernova remnants in the uniform interstellar medium is presented. Some characteristics of nonthermal and thermal emission (luminosity and surface brightness distribution) are compared. The conditions when nonthermal component of X-ray emission may be prominent are revealed. Observational tests for a number of parameters characterizing the cosmic ray injection on the supernova remnant shocks are presented. The cases are considered when the electron radiative losses may be neglected. supernova remnant,X-rays