InxGa1–xN ternary alloys are very promising for a variety of applications. However, high-quality growth of InxGa1–xN alloys, particularly in the intermediate In composition range, is very difficult. This study reports on a systematic analysis of the Raman spectra from the InxGa1–xN alloys grown by radio-frequency molecular beam epitaxy (RF-MBE) for the whole In compositional range, particularly in the intermediate range of In composition. The B1 (high) mode, which is inherently Raman inactive is observed for the InxGa1–xN alloys grown by RF-MBE. The behaviour of Raman inactive B1 (high) mode is studied for the evaluation of InxGa1–xN quality, which is found to vary with the In composition and the temperature of growth. The crystallinity of the InxGa1–xN alloys can be assessed using B1 (high) mode’s relative signal intensity and full-width at half-maximum, which are well agreed with the reflection high energy electron diffraction and X-ray diffraction analyses. The optimum growth temperature for the InxGa1–xN alloys grown by RF-MBE in the intermediate range of In composition is also discussed.