Ni_0.5Mn_0.5−x Sb _x (x = 0.1, 0.2, 0.3, and 0.4) ingots fabricated by arc-melting high-pure metals have been studied to determine their structure and magnetic properties and to identify the magnetocaloric effect. X-ray diffraction analyses reveal that the sample with x = 0.2 is close to a single phase in a cubic structure while the others have the secondary phases. This influences strongly the magnetic properties of Ni_0.5Mn_0.5−x Sb _x . With increasing Sb content, the Curie temperature (T _C ) increases from ∼210 (for x = 0.1) to 435 K (for x = 0.4). Among the studied alloys, two samples, x = 0.2 and 0.3, have the greatest saturation magnetization values, which were recorded at 300 K. Under an applied field of 12.0 kOe, the maximum magnetic entropy changes are about 1.0 and 0.5 J.kg⁻¹·K⁻¹ for x = 0.2 and 0.3, respectively. Detailed analyses related to isothermal M-H curves in the vicinity of T _C by using the modified Arrott method reveal that these samples undergo a second-order phase transition with critical exponents of β = 0.40 ± 0.01 and γ = 1.27 ± 0.08 for x = 0.2 and of β = 0.69 ± 0.09 and γ = 0.85 ± 0.10 for x = 0.3. The differences in the critical parameters are likely related to the presence of Ni-related secondary phases.