Magnetic properties of Cu-doped LaMn1-x Cux O3 (x =0.05-0.30) systems are carefully studied in the temperature range of 2-300 K. A visible unexpected drop is observed in the ac susceptibility and the zero-field cooled dc magnetization curves for the dilute x ≤0.10 near 100 K, which depends on the measuring frequency and magnetic field. Measurements on frequency dependence of ac susceptibility, observation of magnetic relaxation, and the existence of critical field indicate that the anomaly can be attributed to the domain wall pinning effects. This is directly proven by the results of ball milled nano-sized powder counterparts compared with the bulk materials.