Microfibrillated cellulose (MFC) and nanofibrillated cellulose (NFC) isolated from cotton linters were evaluated as a strength additive in unbleached kraft paper and compared with semi-empirical and mechanistic models. The z-directional tensile strength was enhanced due to NFC and MFC. The tensile energy absorption (TEA) derived via integrating the z-directional stress-strain curve was 29.165 J/m2, 120.658 J/m2, and 187.768 J/m2 for the control, MFC, and NFC, respectively. Burst factor significantly increased from 11 to 14 for 10% MFC, while no increase was observed in NFC. From TEA predictions by semi-empirical models, a modified Page equation, Shear-lag, and a negative trend was found due to increased relative bonded area (RBA) with the addition of MFC/NFC. The mechanistic model used six mechanisms involved in binding the fibers and predicted the increased trend of TEA. The increased TEA due to NFC contributed to z-directional tensile strength, but not to the tensile indices and tear factor. This was ascribed to the large size difference of NFC with base pulp fibers and a higher RBA.