The catalytic activity of nanocrystal catalysts depends strongly on their chemical composition, size, and shape. Herein, we report four different sizes and shapes of MnFe2O4 nanoparticles (NPs) prepared by a hydrothermal procedure. In addition, the size- and shape-dependent peroxidase-like activity of these NPs was first explored using 3,3',5,5'-tetramethyl-benzidine and H2O2 as peroxidase substrates. The results showed that the peroxidase-like activities of the MnFe2O4 NPs were size- and shape-dependent and followed the order of 4 nm (spherical) > 18 nm (plate-like) > 27 nm (near-cubic) > 16 nm (spherical); this order was closely related to their surface-to-volume ratio and atom arrangements. Such an investigation is of great significance for peroxidase nanomimetics with enhanced activity and utilization. Furthermore, folic acid (FA)-conjugated MnFe2O4 NPs allow the detection of folate receptor-rich cancer cells. Such investigation can be widely utilized for the identification of important target molecules.