An organic triradical, BIPNNBNO [3,5-bis(N-tert-butylaminoxyl)-4′-(1- oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)biphenyl] includes an antiferromagnetic spin pair of S 1 and S 1/2 within a molecule. In the crystals, ferrimagnetic chains are formed, accompanied by the antiferromagnetic interactions between the nearest and the next nearest neighbouring chains. These interactions induce frustrated spin structure in two-dimension. X-band and submillimeter EPR measurements of BIPNNBNO single crystals were performed. The change of the g-factor and linewidth was observed corresponding to the susceptibility and magnetization behaviour.