Organic magnetic molecules are potential candidates to act as spin filters owing to the spin degeneracy caused by unpaired electron. Based on density functional theory in combination with non-equilibrium Green’s function (NEGF-DFT) method, we study the spin-filtering capability employing the much cultivated and stable organic radicals nitronyl nitroxide (NN) and oxo-verdazyl (OVER). The spin-resolved electronic transportations across the organic radicals connected to the framework of pentacene is investigated by bridging the radical–pentacene moiety between gold electrodes. Unexpectedly, both the stable radicals do not act as efficient spin filters due to the orthogonal arrangement of radical moiety w.r.t pentacene core. On the other hand, the planar arrangement of the radical attached to the pentacene via ethynyl spacer enhances the spin polarization due to much larger delocalization of spin density on pentacene core. Further, the introduction of methyl radical provides an alternative way to use such stable radicals for efficient spin filtering.