In the present report we present the results of our combined experimental and theoretical studies of the trinuclear {[L2Co2Tb][NO3]} (LH3=(S)P[N(Me)N=CH-C6H3-2-OH-3-OMe]3 cluster manifesting single molecule magnet properties. The theoretical model includes the axial crystal fields acting on the Tb3+ and Co2+ ions, the spin-orbit interaction operating within the ground 4T1 term of the Co2+ ion and the ferromagnetic exchange coupling the Tb and Co ions. We demonstrate that for the {[L2Co2Tb][NO3]} cluster the strong negative single-ion anisotropy of both the Tb3+ and Co2+ ions is the reason for the observed slow relaxation of magnetization.figureFig.1. Magnetic susceptibility χT as function of temperature for the {[L2Co2Tb][NO3]} complex (circles - experimental data, solid line - theory). The inset shows the low-temperature part of χT. Fig.2. Low temperature magnetization of the trinuclear heterobimetallic {[L2Co2Tb][NO3]} complex (shapes – experimental data, solid line theory).The elaborated model well describes the observed magnetic susceptibility of the cluster as a function of temperature (Fig.1) and the magnetization as a function of the external magnetic field (Fig.2). Based upon this study, recommendations are formulated on how the SMM properties for 3d-4f-3d clusters may be further improved.