The　macroscopic　fundamental　diagram　(MFD)　for　an　urban　area　provides　a　new　thought　on　aggregate　traffic　management　and　control　that　are　less　affected　by　details.　The　MFD　is　conventionally　constructed　by　using　loop　detector　data,　which　may　not　widely　exist　due　to　the　limited　coverage　of　hardware　installation.　In　contrast,　the　probe　vehicle　data　have　the　benefits　of　low　cost　and　wide　coverage.　To　make　use　of　the　widely　existing　low　cost　data,　we　propose　an　MFD　for　probe　vehicles　(pMFD).　Taking　Beijing　road　network　as　an　example,　we　empirically　demonstrate　that　the　pMFD　is　day-to-day　invariant　in　shape.　We　then　explicitly　analyse　the　relation　between　the　pMFD　and　the　conventional　MFD.　This　study　relaxes　the　data　constraint　of　the　MFD　construction　and　provides　a　new　insight　on　macroscopic　network　condition　monitoring.　©　Published　under　licence　by　IOP　Publishing　Ltd.