After treated by high intensity ultrasonication, a mixture of fiber and fibril in micro and nanoscales was obtained from regenerated cellulose fiber (RCF), pure cellulose fiber (PCF), and microcrystalline cellulose (MCC). Both of the mixture and small fibrils of RCF can be used to increase the tensile modulus and strength of PVA. The small fibrils of PCF and MCC had better reinforcement than that of untreated ones. PLM and AFM observations show that the sizes of the fibrils have a wide diameter range from tens of nanometers to several micrometers; the dispersion of cellulose was uniform from surfaces but not absolutely uniform in the matrix from the cross-sections; and there were some gaps between the polymer matrix and fibrils.