The curing behavior and physical properties of dicyanate/polyetherimide (PEI) semi-interpenetrating polymer network (IPN) systems were investigated. Differential scanning calorimetry (DSC) was used to study the curing behavior of the dicyanate/PEI semi-IPN systems. The curing rate of the semi-IPN system decreased as the PEI content increased. An autocatalytic reaction mechanism can describe well the curing kinetics of the semi-IPN systems. The reaction kinetic parameters were determined by fitting DSC conversion data to the kinetic equation. The glass transition temperature of the semi-IPNs decreased with increasing PEI content. Two glass transitions due to phase-separated morphology were observed for the semi-IPN containing over 15 phr (parts per hundred parts of dicyanate resin) PEI. The thermal stability and dynamic mechanical properties of the semi-IPNs were measured by thermal analysis.