InxGa1−xAs epilayers grown on GaAs(100) were studied by cathodoluminescence (CL) spectroscopy and imaging technique with 0.8 nm spectral resolution, using a transmission electron microscope. Linear features appear in the monochromatic CL image taken by the emission from the InxGa1−x.As layers, and do not appear in those from the GaAs layers. There is no direct correlation between the dark-line contrast in the panchromatic CL image (due to misfit dislocations) and the strong line contrast in the monochromatic CL images of the InxGa1−xAs layers. A peak wavelength shift in the CL spectrum was observed as the electron probe was moved across the linear features. The linear features also appear in a thin sample where the misfit dislocations are removed by ion milling, which clearly reveals that the strong line contrast is not directly due to the misfit dislocation. From those results the linear features in the monochromatic CL image are considered to be due to compositional fluctuations of the In concentration in the InxGa1−xAs layer.