Various hollow manganese oxide (bixbyite Mn 2 O 3 and hausmannite Mn 3 O 4 ) nanoparticles (NPs) with different morphologies were obtained from a single precursor, manganese oxalate IJMnC 2 O 4 ). To synthesize a Mn 3 O 4 stacked nanostructure rather than coral-like Mn 2 O 3 nanospheres, as synthesized MnC 2 O 4 was thermally decomposed at 700°C in the presence of Gd 3+ , through the oriented arrangement mechanism. The formation process and structural variation arising from varying the thermal treatment (450°C and 700°C) and the cationic dopant Gd 3+ were analyzed by FTIR, TGA, and XRD. The unexpected size reduction, and significant physicochemical properties were analyzed using various techniques such as FESEM coupled with EDAX, HR-TEM, DRS-UV-vis, EPR, EIS and VSM. The addition of gadolinium induces particle size reduction and a phase transition from cubic Mn 2 O 3 to tetragonal Mn 3 O 4 , which leads to the suppression of the electrical conductivity, and changes in the optical band gap. The prepared Mn 3 O 4 nanocrystals exhibit ferromagnetic behavior below T c ≈ 45 K and weak paramagnetic behavior at room temperature.