A compact set of equations based on the multiple subbands quasi-ballistic transport theory is developed, and is used to investigate the channel electron effective mobility in recently reported In0.53Ga0.47As/Al2O3 tri-gate n-FinFET. The extracted electron effective mobility μn is around 370 cm2/V·s at low Vg - Vth bias at room temperature and decreases with increasing Vg, and increases with increasing temperature (240-332K). It is very different from the case of Si n-MOSFETs, where the electron mobility decreases with increasing temperature. The low channel effective mobility and the ab-normal temperature dependence of μn are ascribed to the high acceptor interface trap and border trap energy densities in the conduction band energy of InGaAs. The ballistic channel resistance RBall at low Vds is calculated and compared with the measured channel resistance RCH. The low transmission coefficient T = RBall/RCH ≈ 0.06 to 0.05 indicates that there is a large room to improve the InGaAs/Al2O3 n-FinFET performance.