Rubber wood (hevea brasiliensis) sawdust is one of the agricultural waste that have potential in the production of renewable resource such as cellulose and nanocellulose. Utilization of ionic liquids (ILs) as solvents for cellulose processing has been growing exponentially because of the unique physiochemical properties and potential to be ecofriendly. In the present study, 1-buty-3 methylimidazolium (BMIMCl) was used as a solvent to convert cellulose into nanocellulose. The process is done in two steps, there are by isolating α-cellulose from rubber wood through a series of chemical treatments (alkali treatment and bleaching) and converting cellulose into nanocellulose with BMIMCl. The objectives of this study were to determine the effectiveness of BMIMCl to convert cellulose into nanocellulose and to study the properties of BMIMCl treated cellulose. To determine how effective IL treatment is, it was compared to the untreated cellulose. In term of chemical and structural properties, thermal properties and morphological of BMIMCl treated cellulose were characterized through the FTIR, XRD, TGA and FESEM respectively. It is noteworthy that crystallinity and crystallite size of cellulose decreased after IL treatment. In addition, the thermal stability of BMIMCl treated cellulose decreased and the pyrolysis residues increased. The results showed that most crystalline structure of cellulose was destroyed to amorphous structure, and the remained crystalline structure of cellulose was converted to cellulose II from cellulose I in extracted α-cellulose. The more amorphous structure of BMIMCl treated cellulose resulting in a decrease in thermal stability. BMIMCl was effective used to convert cellulose into more amorphous structure. Results from this work may be potentially applied in various fields such as textile, drug delivery and enzyme hydrolysis for bio-energy production.