Net invariants and reachability trees are used to investigate dynamic properties of Petri Nets. Both concepts have been generalized for different classes of High Level Petri Nets. In this paper we introduce the compound token and the token flow path concepts. An algorithm for computing the S-invariants of High Level Petri Nets is presented. In the algorithm, the compound token and the token flow path ideas are adopted and all S -invariants of an HLPN can be generated by a system of integer linear equations without unfolding the net.