In this study, cellulose nanofibers (CNFs), that is, nanosized cellulose fibers, are manufactured from micro-crystalline cellulose (MCC) by using a high-pressure homogenizer. The CNFs are used as reinforcing materials for thermoplastic composites. Polyamide (PA6) and polylactic acid (PLA) fibers are employed as the matrix in the thermoplastic composites. With an increase in the operation pass number and pressure of the homogenizer, the specific surface area (SSA) of the CNFs increases and the crystalline index (CI) decreases. After 30 passes of MCC through the homogenizer, the SSA increases from $257.2m^2/g$ to $787.1m^2/g$, and the CI decreases from 0.78 to 0.70. The tensile strength of the CNF/PA6 composite is higher than that of the CNF/PLA composite. On the other hand, the modulus of the CNF/PLA composite is higher than that of the CNF/PA6 composite. As the CNF content in the composite increases, the total thickness of composite decreases but the tensile strength increases. The CNF/PA6 (3:7) composite has the maximum tensile strength (21.5 MPa) among the samples considered in this study.