Incorporation behaviors of In and Ga of InGaN films grown by the two-heater metal-organic vapor-phase epitaxial horizontal reactor is investigated by varying growth parameters, such as the substrate temperature, ceiling temperature and reactor pressure. Two In loss mechanisms are observed by the analysis of the concentration and temperature profiles in the deposition zone. The gas-phase parasitic-loss mechanism (activation energy 34.2 ± 0.1 kcal/mol) is significant in the ceiling temperature Tceil ≥ 800° C and at substrate temperature (Tsub ) of 600° C. The decomposition-loss mechanism, which arises from In desorption on the growing surface, is significant in Tsub > 625° C region. The desorption activation energy obtained is 28.0 ± 0.1 kcal/mol, which is close to that of InN (25-26 kcal/mol). This suggests the In decomposition-loss mechanism in InGaN growth does not differ substantially from that in binary InN growth. On the other hand, the gas-phase parasitic-loss mechanism of Ga is negligible in the growth condition that we have explored. Exception is found for reactor pressure at Tceil = 950° C, where both factors advantageous and disadvantageous to Ga incorporation are unambiguously observed. We have proposed explanation for the above observation.