A correct estimation of plasma parameters in fusion experiments cannot be done unless the role of polarization of x-ray line emission is recognized.An x-ray polarimeter is presented, based on single hexagonal crystal. The polarimeter selects two polarized components simultaneously. Simple analytical formulae are suggested to determine Bragg angle for analysed x-rays and for the directions of reflected polarized components. The polarimeter is expected to be especially useful to study polarization of x-rays emitted from plasmas with thermonuclear parameters.