A thermodynamic analysis on the metalorganic vapor-phase epitaxy (MOVPE) of InxGayAl1-x-yN quaternary alloy is performed. The equilibrium partial pressures of gaseous species at the deposition zone are calculated for various growth conditions. Then, the deposition modes, growth or etching, and the relationship between solid composition, x and y, and input mole ratios of group III metalorganic sources are studied. It is shown that the growth mode of InGaAlN strongly depends on growth temperature, hydrogen partial pressure in the carrier gas and input group III partial pressure. Complex behavior between the input mole ratios of group III metalorganic sources and the alloy composition of grown InxGayAl1-x-yN is also clarified. Based on the completed deposition diagrams, we can find out the appropriate growth conditions for the MOVPE growth of InxGayAl1-x-yN.