Based on the spin diffusion theory and the Ohm's law, we theoretically studied the spin polarized injection and transport through the ferromagnetic/organic semiconductor/ferromagnetic system and obtained the current spin polarization, which takes into account the special characteristics of organic semiconductors. Self-trapped states, such as spin polarons as well as spinless bipolarons are assumed to be the main carriers in organic semiconductors. From the calculation, it is found that polarons are the effective spin carriers in spin dependent injection and transport in organic semiconductors. The effect of the spin related interfacial resistances, conductivity match and organic semiconductor length on the current spin polarization are also discussed respectively.