A new series of tricyclic pyrimidoquinoxaline derivatives were synthesized and evaluated as antitumor assays and compared with standard drug 5‐fluorouracil. These new pyrimidoquinoxaline derivatives were synthesized by the reaction with o‐aminonitrilequinoxaline derivative 3 with various reagents. One from which, the condensation of o‐aminonitrile with potassium cyanate in acetic acid was stated as