The structural and magnetic properties of perovskite oxides La 0.7 Ca 0.3−x K x MnO 3 (0≤x≤0.15) have been investigated to explore the influence of the A-site cation size-disorder (σ 2 ). The materials were prepared by the solid-state method and then characterized by X-ray diffraction (XRD). The XRD data have been analyzed by Rietveld refinement technique. For K doping concentration x≤0.075, the samples crystallize in the orthorhombic structure, while for x≥0.1, the structure becomes rhombohedral. The variation of the magnetization M as a function of the applied magnetic field μ 0 H reveals the presence of a structural distortion leading to a reduction of the magnetization at low μ 0 H values. When increasing μ 0 H, the structural distortion decreases and for a high applied magnetic field, the M (μ 0 H) curves saturate indicating the disappearance of the structural distortion. The influence of K doping concentration and the applied magnetic field on the magnetocaloric properties has been considered. A large magnetic-entropy change (|ΔS M |∼5J/kgK) is obtained in all samples at Curie temperatures between 270 and 280K for an applied magnetic field of 3T. These results show that these materials can be used as candidates for magnetic refrigerants near room temperature.