Magnetic properties and martensitic transformation of Pd 2 Mn 1+ x Sn 1− x magnetic shape memory alloys have been investigated by first-principles calculations using the pseudopotentials plane wave method. The formation energy results indicate that the excess Mn atom tends to occupy the Sn site in Pd 2 MnSn alloy due to the lowest formation energy. It is found that the antiparallel Mn Sn −Mn Mn magnetic coupling for x = 0.25 and 0.5 is slightly more stable compared to the parallel coupling. The tetragonal distortion conserving volume shows that the cubic structures for x = 0 and 0.25 are more favorable in energy than the tetragonal phases, whereas the tetragonal structure for x = 0.5 is more stable, indicating that a martensitic transformation from austenite to martensite for x = 0.5 may happen at a lower temperature. Moreover, the martensitic transformation for x = 0.5 is closely related to the enhanced Pd−Mn Mn hybridization.