Some basic requirements about measuring differential reflectivity factor are discussed concerning the dualpolarization Doppler weather radar operated under the mode of simultaneous transmission and reception of horizontally and vertically polarized waves. Three kinds of factors, which may induce errors during measuring differential reflectivity factor, are described: (1) the basic requirements for radar measuring cannot be met; (2) echo power fluctuation may result from receiver noises and the relative motion of scatter particles; (3) the attenuations for horizontally and vertically polarized waves differ. Various factors and their weights for influencing the measuring errors of differential reflectivity factor are studied in detail.