This study investigates the rheological properties of plasticized cellulose acetate (PCA) biopolymer and its kraft and microcrystalline cellulose (MCC) reinforced biocomposites. Rheological analyses were performed using a parallel plate rheometer in frequency sweep mode within the linear viscoelastic range. The results revealed molecular interactions between PCA and cellulose fibers, facilitated by their hydroxyl groups, significantly enhancing the biocomposites’ thermal stability. The incorporation of cellulose fibers notably increased the dynamic moduli G′ and G″ and the complex viscosity of the biocomposites at 180 ℃. Higher fiber content resulted in a more elastic behavior. The biocomposites showed shear-thinning characteristics, with complex viscosity increasing by 570 and 100 times compared to neat PCA for biocomposites containing 50 wt% kraft and MCC fibers, respectively. These findings highlight the importance of cellulose reinforcement in enhancing the biocomposites’ mechanical and rheological properties and their potential applications in various fields.