High purity crystals of Y2FeMnO6 (YFMO) and Y2CrMnO6 (YCMO) have been synthesized for the first time using a flux method under a high temperature of 1573 K and high pressure of 6 GPa. Both YFMO and YCMO have orthorhombic structures in space group Pnma. The temperature-dependent magnetization and the nonlinear M-H hysteresis loops of both materials indicate that an antiferromagnetic transition occurs at the Néel temperature of 328 K for YFMO, and a ferrimagnetic transition occurs at 74 K for YCMO. YFMO is a relaxor ferroelectric in which three dielectric relaxations were observed at 245, 328 and 358 K, respectively. The first relaxation process is due to Maxwell-Wagner polarization at the grain boundary whereas the second and the third relaxation behaviours arise from the beginning and the ending of antiferromagnetic ordering, respectively. The presence of the dielectric anomaly near TN indicates the magnetoelectric effect. Ferroelectric hysteresis loops and PUND (positive-up & negative-down) pulse data reveal weak ferroelectric behavior of YFMO at 77 K. Otherwise one dielectric constant anomaly has been found for YCMO at about 390 K. But no ferroelectric polarization was observed in YCMO.