Chiral nematic cellulose nanocrystal (CNC) photonic films with structure color were prepared by regulating the acid-to-pulp ratio (8:1, 10:1, and 12:1) and the hydrolysis temperature (45 degrees C and 60 degrees C). CNC prepared with a lower acid-to-pulp ratio and hydrolysis temperature obtained a higher aspect ratio, which made it easy for CNC to contact each other to produce chiral nematic structures and present iridescent film. The as-prepared CNC iridescent films can reflect natural light with a wavelength in the range of 360-695 nm. Consequently, the CNC films showed structural color of blue to orange, dependent on the acid hydrolysis conditions. The strategy presented herein paves the way for the simple and cost-efficient preparation of photonic materials with structural colors.