Abstract: The crystal mosaic spread determination method is based upon the least square fit of the wavelength distribution of the transmitted neutrons through a slab of a single crystal with the measured one. The method was applied for a crystal cut along (hckclc) lattice plane when it is oriented by an angle ψ with respect to the incident neutron beam direction and satisfying a single Bragg reflection from a certain (hkl) plane. A computer code CRYSHAPE is developed for calculation and fit shape analysis. The deduced crystal angular mosaic spread value was found to be within accuracy of 1.2 min of arc. The method was applied to determine the mosaic spread value of lead single crystal cut along its (311) plane and zinc (002) plane. The measurements of the transmitted neutrons were performed using both time-of-flight and fixed-angle scattering spectrometers, installed in front of the of the ET-RR-1 reactor horizontal channels.