Isotopic substitution can effectively tune the device performances of organic semiconductors. According to the experimental results of isotope effects in electric, light and magnetic process in organic semiconductors, we adopt the tight-binding model with strong electron-phonon coupling to study the isotope effects on carrier transport. We try to give a quantificational explanation and show the physical origin of isotope effects on mobility in organic semiconductors in this work. Using polaron transport dynamics with diabatic approach, we simulate the carrier transport in an array of small molecule crystals under weak bias. Because of strong electron-phonon coupling in organic materials, an injected electron will induce lattice distortion, and the carriers are no longer free electrons or holes, but elementary excitations such as solitons, polarons or bipolarons. Our simulation results indicate that the existence of deuterium and ¹³C element will reduce the mobility of organic material, which means that the isotopic substitution can be utilized to manifest organic device performance. Besides, we also find that the isotope effect on mobility will increase with electron-phonon coupling increasing. This suggests that both the mass of lattice groups and electron-phonon coupling should be taken into account to understand the isotope effects in organic semiconductors. With the consideration of that, we derive the effective mass of polaron based on the continuum model, and verify that effective mass can successfully describe the isotope effect on mobility. The effective mass of carrier can be measured to represent the property of a material, which can tell us whether we need the isotopic substitution in organic layer to improve the device performance. Then we present the microcosmic movement of a polaron at the moment when it encounters isotopic substituted molecules. We come to the conclusion that the isotopic distribution will affect the instantaneous speed of the carrier, but has little effect on the mobility of the whole device when the substituted concentration remains constant. In conclusion, after simulating various possible isotope effects in materials, analyzing its physical mechanism and comparing calculation results in experiment, we provide a theoretical foundation for describing the isotope effects on mobility, which can be a basis of improving the performances of organic semiconductor devices.