properties of the representation is analyzed where we find that the particle-hole symmetry in the spinon Hilbert space of S= 1 fermion representation is absent for S 1 . As a result, different path-integral representations and mean-field theories can be formulated for spin models. In particular, we construct a Lagrangian with restored particle-hole symmetry, and apply the corresponding mean-field theory to one-dimensional 1D S=1 and S =3/2 antiferromagnetic AFM Heisenberg models, with results that agree with Haldane’s conjecture. For a S=1 open chain, we show that Majorana fermion edge states exist in our mean-field theory. The generalization to spins with arbitrary magnitude S is discussed. Our approach can be applied to higher dimensional spin systems. As an example, we study the geometrically frustrated S=1 AFM on triangular lattice. Two spin liquids with different pairing symmetries are discussed: the gapped px+ipy-wave spin liquid and the gapless f-wave spin liquid. We compare our mean-field result with the experiment on NiGa2S4, which remains disordered at low temperature and was proposed to be in a spin-liquid state. Our fermionic mean-field theory provide a framework to study S 1 2 spin liquids with fermionic spinon excitations.