This work reports on the mechanical and thermal properties of poly(vinyl alcohol) (PVA) nanocomposites reinforced with nanocelluloses isolated by the sulfuric acid hydrolysis using commercial microcrystalline cellulose (MCC). Nanocellulose-reinforced PVA nanocomposite films were prepared by the casting method with different nanocellulose loadings, which were exposed to tensile test, thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). The nanocellulose obtained by the acid hydrolysis was a rod-like whisker form. Wet particle size analysis resulted in an average size of about 340 nm. But, the measurements of individual nanocelluloses using transmission electron microscopy (TEM) provided the dimensions of about 6.96 nm wide and 178 nm long. The crystallinity of the nanocellulose was quite high (85.2%), which was greater than that of the MCC. The tensile modulus and strength of the nanocomposites were improved with an increase in the nanocellulose content, but decreased at the nanocellulose content of 7 wt%. Thermal stability of the nanocomposites was improved as the nanocellulose content increased up to 7 wt%. The DMA result shows a significant increase of the storage modulus of the nanocomposite at the 3 wt% nanocellulose. These results indicate that the nanocellulose has a great potential to reinforce PVA polymers.