The Y3Co11-xMxB4 with M=Cu or Al form solid solutions in the composition range x less than or equal to0. The compounds are ferromagnetically ordered for x less than or equal to3 (M=Cu) and x less than or equal to1.5 (M=Al). In case of Y3Co11-xAlxB4 system for x greater than or equal to2, the compounds are not magnetically ordered. The cobalt magnetic moments show a sudden increase for an exchange field around 50 T. Then, the cobalt magnetic moments increase with increasing exchange fields according to the relation DeltaM(Co)=V-Co*DeltaH(exch) where V-Co=(2.2*10(2))(-1) mu (B)/T. Finally, the magnetic behavior of cobalt is discussed in spin fluctuations model.