Starch-based composites have become promising materials for eco-friendly packaging applications because of their biodegradability and cost-effectiveness. However, the inherently poor mechanical and barrier properties of starch-based composite films hinder their market potential. Here, nanocrystalline cellulose (NCC) was isolated from cotton cellulose powders using sulfuric acid hydrolysis. Starch-based composite suspensions/films reinforced with 0.10.5 wt % of NCC were prepared and characterized. Surface sizing application of the reinforced composite suspensions on cellulosic paper was subsequently studied for improving the mechanical properties and the resistance to air permeability of cellulosic paper. Results revealed that NCC reinforced composite suspensions showed the characteristics of non-Newtonian fluids. The rheological behavior of these suspensions showed strong NCC concentration dependence. NCC addition improved the thermal stability of starch-based composite films. In addition, both the mechanical properties and air permeability of surface-sized paper tended to achieve an optimal state when NCC addition was 0.3 wt %.