In view of the concept of circular economy and the zero waste approach agricultural residues can be viewed as sustainable and viable biomass resources for nanocellulose production. Therefore, this paper presents a method of obtaining cellulose nanocrystals (CNC) from non-invasive and fast-growing grass genera, Miscanthus (M. sacchariflorus and M. sinensis) and Sorghum (S. saccharatum and S. bicolor). The cellulose isolated from this biomass was solvolyzed with ionic liquids: 1-allyl-3-methylimidazolium chloride – AmimCl and 1-ethyl-3-methylimidazolium acetate – EmimOAc. Nanometric size of obtained material (average diameter ranged from 27 to 54 nm) was confirmed by the scanning electron microscopy (SEM). The X-ray diffraction (XRD) analysis indicated the transformation of solvolyzed material from cellulose I into cellulose II, which was especially visible for the material treated with EmimOAc. The crystallinity index (Xc) of all cellulose nanocrystals (0–26%) was lower than that for initial cellulose (29–41%). The properties of CNC obtained from Miscanthus and Sorghum biomass depend on the type of ionic liquid used in the solvolysis process and the type and species of the plant from which initial cellulose was isolated. The obtained results suggest that cellulose isolated from plant sources, such as Miscanthus and Sorghum biomass can be used as a material for the production of CNC by ILs solvolysis.