Novel polymers obtained by reaction of aromatic biscyanamide and bismaleimide compounds were investigated. By heating 4,4′-methylene bis(o-methylphenylcyanamide) (BMCA) with 4,4′-methylene bis (phenylmaleimide) (BMI) at 120°C in N,N-dimethylformamide (DMF), prepolymers were obtained by the conversion of cyanamide to cyanoguanidine and the addition of an imino group on the double bond of maleimide. The prepolymer showed suitable behavior for thermosetting resin used in molding, i.e., it melted temporarily and then polymerized in an isochronal heating process (5°C/min). On heating above 170°C, the prepolymer could polymerize with ring formation of melamine and isomelamine. The cured product had good heat-resistant properties.