In　this　paper,　we　study　M-estimation　for　the　partially　linear　model　under　monotonic　constraints.　We　use　monotone　B-splines　to　approximate　the　monotone　nonparametric　function.　We　show　the　large　sample　properties　of　the　resulting　estimators.　The　proposed　estimator　of　parameter　part　is　root-n　consistent,　and　asymptotically　normal　and　the　estimator　for　the　nonparametric　component　achieves　the　optimal　convergence　rate.　A　simulation　study　is　conducted　to　evaluate　the　finite　sample　performance　of　the　method.　The　proposed　procedure　is　illustrated　by　an　air　pollution　study.　Crown　Copyright　(C)　2013　Published　by　Elsevier　B.V.　All　rights　reserved.