Twinned and detwinned Yba2Cu3O7−y (Y123) single crystals were observed by transmission electron microscopy to investigate the influences of the lattice defects on the peak effect for Y123 single crystals. Twin boundaries with the spacing of 0.2–0.5 μm were seen only in the twinned Y123 single crystals. Antiphase boundaries (APBs) along the (110)Y123 plane were observed in both twinned and 0.5h-detwinned samples, which showed the peak effect in high magnetic field over 3 T at 70 K. In contrast, APBs were not found in 100h-detwinned sample. Critical current densities of this sample were small all over the range of the applied magnetic field. In conclusion, we suggest that APBs have a significant influence on the peak effect of Y123 single crystals in the high magnetic field.