Spinel ferrite thin films Fe_3-x Mn_xO₄ (x=0.000, 0.006, 0.0010, 0.015, 0.023) were prepared on the coverglass by ferrite plating technique. To investigate the uniaxial anisotrpy of the samples, the saturation and effective magnetization of the thin films were measured by VSM(vibrating sample magnetometer) and FMR(ferromagnetic resonance) measurements respectively. The spectroscopic splitting g factor were estimated from the ferromagnetic resonance curves. For x=0.000, 0.006, the effective magnetization was measured of temperatures form T=77 K to T=300 K. The results were analyzed in terms of Bloch's law M_s(T) = M_s(0) (1-BT^3/2-CT^5/2). The Bloch coefficient B, C were determined by fitting. M_s(0) was obtained by extrapolating M_eff to 0 K. From this result, the spin wave stiffness constants D was also determined.