Isopropanol-butanol-ethanol (IBE) can be utilized as a clean transportation fuel to avoid the production cost of biobutanol as well as the plastic-degradation issues of acetone-butanol-ethanol (ABE). In light of this, optical experiments were conducted in a constant volume chamber under various test conditions to reveal the spray, combustion, and flame structure of IBE/diesel blends. The experimental results show that with the addition of IBE into diesel, the spray evaporation improves significantly; the larger the IBE blending ratio is, the shorter the liquid spray penetration. However, the addition of IBE causes the maximum combustion pressure and peak apparent heat release rate to reduce. Compared with diesel, the flame lift-off length (FLOL) of the fuel blends is noticeably longer. Furthermore, it prolongs as the amount of IBE in the blend increases. The long FLOL of the fuel blends combined with its short liquid spray penetration contributes to a large mixing area for fuel to evaporate, which reduces the local equivalence ratio. Results also show that the fuel blends always give a lower spatial integrated natural luminosity (SINL) than diesel. The SINL curve gradually gets narrower as the IBE blending ratio increases. That is to say, the addition of IBE into diesel can reduce the rate of soot formation but accelerate the soot oxidation.