This paper describes a comparison of the optical properties of InGaN:Zn with that of GaN:Zn and InGaN by measuring photoluminescence excitation (PLE) spectra at 77 K. It is well known that MOCVD grown InGaN films tend to have a fluctuation in In concentration which results in a fluctuation of the band gap energy. The PL mechanism in InGaN films has been assigned to the annihilation of an exciton at the potential minima caused by the fluctuated band gap potential. We grew InGaN(:Zn) and GaN:Zn microcrystals emitting intense blue luminescence by a reaction of GaN and In2S3 with NH3 in the range of 850 to 900 °C. The samples grown at various temperatures show two PLE peaks: one weak peak is located around 3.47 eV, which we attribute to the band gap energy, and the other peak around 3.15 eV, which we attributed to the In localized state level. We had proposed an atomic structure of the localized state based on an isoelectronic trap theory. However, it is necessary to estimate the order of potential fluctuation of the grown InGaN microcrystal is small in order to assure the isoelectronic trap theory. PLE spectra of InGaN:Zn were measured and compared with that of GaN:Zn to estimate the degree of energy gap fluctuation. As the shape of a PLE peak of InGaN:Zn at around 3.47 eV was comparable to that of GaN:Zn, we concluded that the isoelectronic trap model holds for the grown InGaN microcrystals.