A straightforward approach to novel fused beta-carbolines namely [1,3,5]triazino [1',2':1,6]pyrido[3,4-b]indol-4-ones is presented. The construction of these compounds was achieved by one-pot synthesis involving condensation of 3-amino-beta-carbolines with ethoxycarbonyl isothiocyanate followed by amination of the resulting thioureas and finally thermal ring closure of the resulting guanidines which allowed access to the unreported title heterocycles.