The spontaneous relaxor-normal phase transition which occurs at PYN-rich composition in $(1-x)PbYb_{1/2}Nb_{1/2}O_3 - xPbFe_{1/2}Nb_{1/2}O_3$ solid solution system has been investigated by dielectric constant measurements, X ray diffraction, P-E hysteresis loop measurements, transmission electron microscopy.
From the dielectric constant measurements, the spontaneous relaxor-normal transition was observed near x=0.14. Here the driving force of the relaxor-normal transition of x=0.14 system is considered to be the increase of elastic strain energy resulting from the existence of antiferroelectric microdomain with locally orthorhombic symmetry in cubic paraelectric matrix. Through the analysis of all the experimental results, 0.86PYN-0.14PFN ceramic is found to undergo a successive phase transition, antiferroelectric - ferroelectric - paraelectric, in the order corresponding to increasing temperature. Here each phase boundary is not distinct and the coexistence of related phases was observed.