MnCoGe　is　one　of　the　most　representative　materials　that　undergoes　martensitic　transformation　in　the　MM′X　system.　Because　the　phase　transition　temperature　of　MnCoGe　is　higher　than　its　magnetic　ordering　temperature,　various　methods　have　been　used　to　decrease　the　phase　transition　temperature　and　realize　magneto-structural　coupling.　However,　there　are　rare　reports　about　the　effect　of　doping　with　rare-earth　elements.　In　this　work,　the　structures　and　physical　properties　of　Mn1−xLaxCoGe　compounds　with　x=0,　0.01,　0.02,　0.03,　and　0.05　were　studied.　The　introduction　of　La　causes　a　deficiency　of　both　Mn　and　Ge　atoms　in　the　main　phase,　which　leads　to　a　decrease　in　the　martensitic　transformation　temperature　and　the　realization　of　magneto-structural　coupling.　However,　due　to　its　limited　ability　of　deficiency　formation,　a　further　increase　in　the　La　content　leads　to　large　amount　of　secondary　phase　instead　of　a　continuous　decrease　of　the　phase　transition　temperature.　Additionally,　the　MnCoGe　matrix　is　found　to　be　divided　into　small　“domains”　by　the　needle-like　secondary　phase,　which　is　the　reason　for　the　large　amount　of　retained　austenite　phase　in　the　sample　with　a　high　La　content.　©　2021.