We report on the magnetization, pinning force, and magnetic relaxation of twinned and detwinned NdBa2Cu3Oy (NBCO) single crystals. A double peak feature appears in the magnetization curves of the crystal having one type of twin boundary (TB) only in one direction, whereas a single peak is observed in crystals either without twins, or with microtwins or mixed twins in both directions. The double peaks disappear when the magnetic field is tilted away from the c axis of the crystal removing the influence of TB's. Tn the high field region, due to suppression of thermal fluctuations by the presence of TB's, the relaxation rate in twinned NdBCO crystal is small, and the pinning force density is increased compared to the YBCO and detwinned NdBCO crystals. [S0163-1829(98)09038-9].