A system of (1 − x)Pb(Yb1/2Nb1/2)O3 (PYN)–xPb(Mg1/3Nb2/3)O3 (PMN) (0 ≤ x ≤ 1) has been investigated with regard to its phase transition, densification, and dielectric and electrostrictive properties. According to the XRD study, a crystal structure transformed from orthorhombic to pseudocubic at approximately x = 0.22. The superlattice peaks were gradually weakened with increasing x, and disappeared above x = 0.22. Increasing x led to an increase in the maximum dielectric constant and a decrease in transition temperature over the entire composition range. As a result of P-E hysteresis loops, successive phase transitions of ferroelectric-antiferroelectric-paraelectric were observed to occur in the range of 0.18 ≤ x ≤ 0.25 with increasing temperature, while the direct transition into the paraelectric region was found to take place for x ≥ 0.3. From the field-induced strain measurement, high electrostrictive coefficients, 7.3–8.2 × 10−2 (m4/C2), were determined in the PYN-rich range (x ≤ 0.1). Based on the results, a phase diagram of the system was constructed with variations in x and temperature.