Cellulose nanocrystals (CNCs) were successfully extracted from wood flour by a two-step process that comprised ethanol and peroxide solvothermal pretreatment and an ultrasonic disintegration process. Characterization results showed that 97% of the total lignin and 70% of the hemicellulose could be fractionated in a single ethanosolv pretreatment step. Additional treatment with alkaline hydrogen peroxide removed the residual lignin and hemicellulose and resulted in high purity cellulose. The CNCs obtained after ultrasonication displayed a similar yield, size, morphology, and crystallinity but had better thermal stability and film forming properties than those produced by concentrated acid hydrolysis. Overall, the solvothermal treatment using ethanol and its combination with peroxide is an ideal substitute method for pretreatment of lignocellulose. Further integration of such pretreatments with ultrasonication provides a promising efficient process with low environmental impact for production of CNCs.