Wood is currently the most used source of cellulose [27,28]. Moreover, cellulose is present in wood in combination with hemicellulose, lignin, and a comparably small amount of extractives and inorganic salts. Therefore, wood must be treated by mechanical and/or chemical processesto remove noncellulosic material before being used for nanocellulose production. Wood species can be distinguished as hard and softwoods based on their anatomical features. Hardwood fibers are 34 times shorter than softwood and have more rigid structure due to their high Runkel ratio (cell wall thickness divided by lumen radius) [29]. Moreover, hardwood is more complex and heterogeneous in structure than softwood, thus requiring harder mechanical treatment to produce equivalent fibrillation level [30].It should be also noted, that the use of neverdried cellulose makes the fibrillation more favorable compared to once-dried cellulose, since drying promotes irreversible hydrogen-bonding between nanofibrils. This process is known as hornification [31,32].