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We investigate the effects of off-stoichiometric compositional variations from the Co2Fe(Ga0.5Ge0.5) (CFGG) full-Heusler alloy on its half-metallic electronic structure. First-principles calculations predict that the Co antisite defects that occupy Fe-sites (CoFe) lead to a finite DOS in the half-metallic gap of CFGG. Fe antisites defects in Co-sites (FeCo) introduced by excessing Fe composition, which could suppress the formation of CoFe, preserves the half-metallic gap but reduces spin polarization because the Fermi level shifts to the lower energy. We found that, in Fe-excess CFGG, Ge-excess has an important role to enhance the spin polarization by lifting up the Fermi level position and suppressing the formation of CoFe. To confirm the effect of the Fe and Ge-excess off-stoichiometric composition on spin polarization and phase-purity experimentally, we fabricated CFGG epitaxial thin films with various composition ratios (Co2− αFe1+ α) (Ga1− βGe β)1+ γ with small positive γ (=0.09–0.29). It turns out that Co1.75Ge or Fe1.7Ge secondary phase often forms in the films for [Formula: see text] in Fe-deficient [Formula: see text] and excess [Formula: see text] compositions. This secondary phase can be suppressed by tuning the Ge and Fe compositions, and the L21-phase pure film was found in Co39.4Fe29.3Ga13.4Ge17.9 [Formula: see text]. The measurements of conventional magnetoresistance effects qualitatively indicate higher spin polarization in the Co39.4Fe29.3Ga13.4Ge17.9 film compared to other Co-excess and Ge-deficient films, which evidences the benefit to make Fe- and Ge-excess off-stoichiometric CFGG for obtaining the half-metallic nature of CFGG.