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In view of the need to limit harmful emissions of chlorofluorocarbons into the atmosphere and sharp increase in life support areas that require cooling, the search for new magnetocaloric materials is very urgent at the present time. In this work, we suggest the ways for improving the magnetocaloric properties of the promising La 0.8- x K 0.2 Mn 1+ x O 3 (0 ≤ x ≤ 0.2) perovskites and compare the functional properties of nanocrystalline samples of about 40 nm with polycrystalline samples of about 1 μ m. The structure, morphology, magnetic properties and magnetocaloric effect of these compounds were studied as a function of manganese doping level. The main crystalline phase of all samples is a rhombohedral R 3 ‾ c perovskite structure. The maximum doping level of excess manganese for preserving single-phase structure of the nanocrystalline La 0.8- x K 0.2 Mn 1+ x O 3 compounds is x = 0.10. As x increases the Curie temperature T C is increased slightly. The best composition with the highest entropy change Δ S M max = −3.629 J/(kg⋅K) under 3 T at T C = 332 K is the La 0.8- x K 0.2 Mn 1+ x O 3 with x = 0.05. On the other hand, the La 0.8- x K 0.2 Mn 1+ x O 3 polycrystalline manganite with x = 0.10 exhibits the highest Δ S M max = −4.176 J/(kg⋅K) under 3 T at T C = 274 K. Additionally, the effect of hydrostatic pressure P up to 1.4 GPa has been investigated for the La 0.7 K 0.2 Mn 1.1 O 3 polycrystalline sample that leads to increasing T C to 287 K and reducing Δ S M max to −3.145 J/(kg⋅K) under 3 T. The obtained results demonstrate the ways for controlling magnetocaloric properties of the La 0.8- x K 0.2 Mn 1+ x O 3 compounds under internal and external conditions. [ABSTRACT FROM AUTHOR]