To study the phase diagram and dynamics of interlayer vortices in underdoped YBa2Cu3Oy single crystals (T c - 47 K), we measured the c-axis resistivity ρc in magnetic fields H parallel to the ab-plane (H || ab). We find that the field dependence of the resistivity ρc(H) shows the oscillations under high current densities above J 4 A/cm2. In low fields below 3 T, the peaks in ρc(H) agree with the commensurate condition between the intervortex spacing perpendicular to the CuO2 layers and the layer period. In high fields above 3 T, however, peaks in ρc(H) deviate from the commensurate condition. The angular dependence of ρ c(θ) suggests that the deviation originates from the strong pinning effect for interlayer vortices in high field region.