The dependence of the energy band-gap on In atomic fraction for the liquid phase epitaxy (LPE) GaInSb/GaSb multigrading layers has been investigated by infrared transmission measurements combined with electroprobe microanalysis (EPMA). The result showed that epilayers with cut-off wavelength up to 4.71-mu-m at room temperature and with In atomic fraction as high as 0.75 have been obtained for the first time.