The dielectric properties and phase transition behaviors of (1-x)Pb(Yb1/2Ta1/2)O-3-xPb(Yb1/2Nb1/2)O-3 (PYT1-xPYNx) solid solution have been investigated. The primary transition that occurs at 310 degreesC of PYT is not affected by substitution of Ta5+ by Nb5+. The secondary phase transition of PYT which happens to be relaxor-like in character is critically affected by Nb5+ substitution. Although the profiles of X-ray diffraction of the. solid solution are not noticeably changed by Nb5+ substitution, its selected area electron diffraction reveals that the superlattice diffraction spots become incommensurate due to Nb-5+ substitution. This incommensuration. coincides with disappearance of the secondary phase transition on Nb5+ substitution.