Abstract Reducible state variables are important for the study of reduced-order observer and control design of logical control networks (LCNs). This paper studies the number of reducible state variables of LCNs. Based on the upper bound of reducible state variables, several sufficient/necessary conditions are presented for the number of reducible state variables. In addition, using the regular subspace theory, the coordinate transformation as well as the swap matrix are constructed, and some new criteria are proposed to calculate reducible state variables.