The development of cellulose-reinforced biomaterials appears to be an attractive approach for the production of sustainable materials with good mechanical properties. Although 3D printing of cellulose-reinforced biomaterials has become popular, there is very little feedback regarding their use in electrical insulation applications. This study aims to propose bio-nano-composites containing polylactic acid (PLA), microcrystalline (MCC) and nanocrystalline (NCC) cellulose by fused filament fabrication (FFF) for electrical insulation applications. The influence of the 3D printing process, cellulose content and filler size on dielectric properties was investigated. The addition of cellulosic fillers, and considering their high polarity, increased the dielectric constant (ε'), dielectric loss (ε''), as well as the AC electrical conductivity (σAC) of the composites. Cellulosic fillers also increased the crystallization rate of the materials. At equivalent content, the highest polarization potential were observed for NCC-based composites and were attributed to the nanofiller's better dispersion and available specific surface area. Finally, the 3D printing process affects all the measured properties, due to a combined effect (lower crystalline content and voids presence). The porosity rate was measured at 2.5 % for the neat PLA and increased progressively from 3.1 % to 5.8 % for the cellulose-based composites. These findings showed the benefits provided by the FFF technology in the production of cellulose-based biocomposites with good electrical insulation properties, while noting some needed feedback on the thermomechanical properties of such materials.