A phoswich detector was developed to measure neutron energy spectra from a few MeV to a few hundreds MeV in aircrafts and space crafts. Radiation fields, which both crafts are exposured, consist of neutrons, gamma rays, protons, etc. The phoswich detector can measure neutrons separately from gamma rays and protons. The capability of particle discrimination was tested at HIMAC and was found to be excellent. Detector response functions to neutrons were simulated with the MCNPX code using the measured light outputs of charged particles and were measured with quasi-mono-energetic neutrons produced by the p-Li reaction at the NIRS cyclotron. Test flight measurements at high altitudes, 6.5 and 8.5 km, were performed above the middle part of Japan (cut-off rigidity, 12 GV).