Addition of certain fines can improve the tensile strength of paper. Cellulose nanofiber (CNF) and microfibrillated cellulose (MFC) are some of the fines that are often studied in the field of paper science. However, the differences in the retention capacities of CNF and MFC are not well known. Hence, in this study,the effects of adding CNF and MFC for improving the tensile strength of paper were analyzed and their respective retention capacities were compared. Consequently, under presumption that the retention of fines included in the pulp suspension did not alter, the retention ratios of CNF and MFC were found to be approximately 50％and 90％, respectively. Furthermore, in the case of equal retentions, CNF can augment the tensile strength of paper three times more than MFC. As compared to MFC, CNF can significantly improve the bonding strength between pulp fibers.