Place- and transition-invariants of structured operating procedures described by low level Petri nets containing a plant-level and embedded operating unit-level procedures are analysed in this paper. Place-invariants form a set of system states with, conservation property while transition-invariant correspond to cyclical behaviour of the system. It is shown how the place and transition-invariants of the complete operating procedure can be determined from the invariants of the plant-ant unit-level procedures. The results are illustrated on a simple example.