A series of Eu2+- and Mn2+-codoped gamma-AlON (Al1.7O2.1N0.3) phosphors was synthesized at 1800 degrees C under 0.5 MPa N-2 by using the gas-pressure sintering method (GPS). Eu2+ and Mn2+ ions were proved to enter into gamma-AlON host lattice by means of XRD, CL, and EDS measurements. Under 365 nm excitation, two emission peaks located at 472 and 517 nm, resulting from 4(f6)5d(1) -> 4(f7) and T-4(1)(4G) -> (6)A(1) electron transitions of Eu2+ and Mn2+, respectively, can be observed. Energy transfer from Eu2+ to Mn2+ was evidenced by directly observing appreciable overlap between the excitation spectrum of Mn2+ and the emission spectrum of Eu2+ as well as by the decreased decay time of Eu2+ with increasing Mn2+ concentration. The critical energy-transfer distance between Eu2+ and Mn2+ and the energy-transfer efficiency were also calculated. The mechanism of energy transfer was identified as a resonant type via a dipoledipole mechanism. The external quantum efficiency was increased 7 times (from 7% for gamma-AlON:Mn2+ to 49% for ?-AlON:Mn2+,Eu2+ under 365 nm excitation), and color-tunable emissions from blue-green to green-yellow were also realized with the Eu2+ -> Mn2+ energy transfer in gamma-AlON.