This study was aimed at improving the gas barrier property of sugarcane-based LLDPE using cellulose nanocrystals (CNCs). Specifically, this study evaluated the effect of testing methods (isostatic versus gravimetric) on CO₂ permeability coefficient (P_CO2) and/or O₂ permeability coefficient (P_O2) of various bio-PE grades with different densities (LLDPE, LDPE, and HDPE) as well as the effect of CNC content on crystallinity, tortuosity factor, and gas barrier properties of bio-LLDPE sheets and films. The isostatic and gravimetric methods yielded similar P_CO2, irrespective of PE grade. However, the P_CO2 negatively correlated with PE density. All nanocomposites showed considerable improvement in gas barrier irrespective of the CNC content. The P_CO2 of LLDPE sheets decreased by 36% by adding 10 wt.% of CNCs into the sheet. Similarly, a significant decline in both P_O2 (about 50%) and P_CO2 (about 33%) of LLDPE films was obtained by adding 2.5 wt.% of CNCs into the films. Nevertheless, no correlation was established between gas permeability and percent crystallinity of LLDPE sheet since the P_CO2 decreased almost linearly with increasing CNC content whereas the percent crystallinity of LLDPE increased only up to 2.5% CNC content and remained constant thereafter. In contrast, the tortuosity factors calculated from the diffusion coefficients increased almost linearly with CNC contents and correlated well with the gas permeability improvement in the bio-LLDPE-based nanocomposites. Consequently, the enhanced gas barrier in the nanocomposite was assigned to the tortuosity effect created by the impermeable cellulose nanocrystals rather than the changes in percent crystallinity.