Excitation function for the B 11 ( p , n )C 11 reaction was measured by the activation method with improved energy resolution for the proton energies ranging from 4.7 to 15.0 MeV. The excitation function exhibited rather clear peaks at proton energies of 8.5, 9.9, 10.9, 11.5 and 13.6 MeV in the laboratory system and a broad peak at the energy region between 13.8 MeV and 14.5 MeV. A considerable number of uncertain peaks were also observed. The excitation function for the O 16 ( p , α)N 13 reaction was also obtained above E p =9.7 MeV.