Highly porous agglomerates of spray freeze dried cellulose nanocrystals (SFD-CNC) were prepared, starting with sonicated aqueous suspensions of spray-dried cellulose nanocrystals powder (SD-CNC). Subsequently, SFD-CNC together with the SD-CNC (used as a reference) were incorporated into LLDPE and HDPE via melt compounding in a batch mixer to produce nanocomposites containing 0.5 wt.% and 2 wt.% CNC. Differential scanning calorimetry (DSC) was used to study the thermal properties and the isothermal crystallization kinetics of the polyethylenes and the nanocomposites. Polarized light microscopy (PLM) was used to evaluate the growth kinetics and spherulitic structure of polyethylene in both the filled and unfilled polymers. Avrami crystallization kinetics models were employed to analyze the DSC results. It was observed that CNC acts as a heterogeneous nucleating agent in LLDPE nanocomposites, thus yielding nucleation controlled crystallization. On the other hand, in the HDPE systems (polymer and nanocomposites) heterogeneous nucleation was followed by 3-D growth. It was observed that CNC slightly hindered the formation of chain folding for the HDPE, similar to previous studies on the polypropylene and its nanocomposites. Spray freeze drying produced twice as many nucleation sites compared to spray dried samples and it enhanced the overall crystallization rate and the crystallinity.