In this article, we report the substrate effect on ferroelectric and magnetic properties of epitaxial BiFeO3-based thin films at room temperature. (La, Mn) cosubstituted BiFeO3 (BFOLM) thin films were deposited on differently lattice mismatched single-crystal substrates to manipulate the strain states in the as-deposited films. All the films with 30-nm thick CaRuO3 bottom electrodes exhibited highly epitaxial growth behavior with a slightly monoclinic distorted lattice structure while their strain states are drastically different as confirmed by X-ray reciprocal space mapping. These films possessed significantly different macroscopic ferroelectric properties with giant remanent polarization of 101 ± 2, 65 ± 2, and 48 ± 2 μC/cm2 for the films grown on SrTiO3, (La, Sr)(Al, Ta)O3, and LaAlO3, respectively. It is found that the room-temperature magnetic properties are also in accordance with their strain state, having a reciprocal relationship with polarization. For example, the enhanced magnetization is associated with the suppressed polarization and vice versa. The stain tunability of multiferroic properties in BFOLM thin films are presumably ascribed to the polarization rotation and oxygen octahedral tilts.