Crystal structure and dielectric properties of (1-x)Pb(Yb1/2Nb1/2)O-3-xPb(Sc1/2Nb1/2)O-3 (PYN1-xPSNx) (0less than or equal toxless than or equal to0.60) solid solution have been studied, and its pseudobinary phase diagram has been determined. The low-temperature crystal structure of PYN1-xPSNx becomes pseudocubic from the PYN's orthothombic with increasing x. Also the x-ray diffraction line intensities ascribed, respectively, to the antiferroelectric (AFE) lattice modulation and the B-site ordering become gradually weaker and then they disappear altogether for x>0.37. In the PYN-rich composition range (0less than or equal tox<0.37), the paraelectric (PE)-AFE phase transition occurs sharply and its transition temperature remains independent of the applied frequency. However, on further increasing Sc concentration (0.37