In this paper, we report the growth of epitaxial films of ZnGeP2on GaP and Si substrates of (001) and (111) orientations by open tube organometallic chemical vapor deposition. Generally, smaller growth rates are required to achieve similar film quality on (111) GaP as compared to growth on (001). For the growth on (001) GaP thec‐axis strain determined by x‐ray diffraction agrees with the expected strain for a lattice mismatch of 0.0026 between thea‐axis lattice parameters of ZnGeP2and GaP. Based on the continuous increase of thec‐axis lattice parameters and the gradual weakening of the chalcopyrite structure superlattice reflections, a continuous transition is proposed from the ordered structure of ZnGeP2towards diamond structure of Ge via partially disordered metastable solid solution of ZnGeP2‐Ge. Cross‐sectional transmission electron microscopy reveals twinning of epitaxial ZnGeP2‐Ge films on (111) GaP. The twinning density is related to both the growth rate and the flow rate ratio of Zn(CH3)2to GeH4at constant flow rate of PH3and H2. Twins are also formed in ZnGeP2growth on Si substrates.