A pressure-induced ferromagnetic to anti ferromagnetic transition has been found in the polycrystalline sample of the beta -phase crystal of p-nitrophenyl nitronyl nitroxide (p-NPNN). As the pressure increases, the ferromagnetic transition temperature decreases with simultaneous decrease of the ac-susceptibility in the ordered state, Further increase of pressure above about 6.5 kbar results in antiferromagnetic behavior of chi (ac)-T and M-H curves. The observed ferromagnetic to antiferromagnetic transition suggests that one of the exchange paths has changed the sign of the interaction. This is discussed in relation to the structural change under high pressure.