Experiment(E398) was carried out at RCNP(Osaka) to measure the energy spectrum and emission probabilities of gamma-rays from giant resonances of 12C and 16O using 392-MeV proton beam, high-resolution magnetic spectrometer "Grand Raiden" and array of NaI(Tl) scintillators. Gamma-ray emission probability from the giant resonances of 12C and 16O has been measured as a function of excitation energy(Ex). The maximum gamma-ray emission probability was measured to be 70% for 12C at Ex=27MeV and 90% for 16O at Ex=23MeV until the energy threshold for two nucleons decay, then decrease gradually. We also present the hadronic decay model calculations for the transition probability from the giant resonance states to the excited states of daughter nuclei. We compare the calculations with our measured gamma-ray emission probability. Combining E398 gamma branching with 12C(ν,ν') and 16O(ν,ν') cross section ,we also estimate supernova NC neutrino events expected at KamLAND and Super-Kamiokande.