Measurements of isothermal magnetic hysteresis loop in c-aligned external field have been performed on superconducting La1.6-xNd0.4SrxCuO4 (x = 0-10, 0.20) single crystals, resulting in the H-T phase diagrams for the respective samples. The isothermal magnetization data of Sr-0.10 reveal the existence of fishtail effect in the certain temperature range of measurement (2.5 less than or equal to T less than or equal to 5.5 K), terminated at temperature well below the superconducting critical transition temperature (T-c(on) approximate to 8 K). Analysis of the irreversibility line shows a sign reversal of its slope at around the crossover field, HIM, indicating 2D melting at higher field. On the other hand, a single H-irr (T) function of 3D character was found to fit the irreversibility data for the Sr-0.20 sample, indicating the lack of vortex line decoupling effect in the same range. These remarkably different behaviors are suggested to have their origin in the different degrees of anisotropy (gamma approximate to 96 for Sr-0.10 vs. gamma approximate to 25 for Sr-0.20) as a consequence of different levels of Sr doping.