A variety of techniques have recently been developed that enable manufacturing of cellulose nanomaterials with optimal properties for certain applications. Acid catalyzed hydrolysis is currently the most popular method for the synthesis of cellulose nanocrystals (CNCs). However, the type of acids and hydrolysis conditions influence the CNCs thermal stability which is crucial parameter in polymer composite technology. In this work thermally stable CNCs were prepared by two step method - (i) hydrolysis in phosphoric acid solutions to produce fine microcrystalline cellulose (MCC), (ii) followed by modification of MMC with succinic anhydride (SA). The method allowed manufacturing CNC-SA material that showed higher thermal stability than the reference cellulose material (UFC-raw) both in the inert gas and air atmosphere. The kinetic parameters of the thermal degradation process of produced cellulose materials were determined by Friedman and Ozawa-Flynn-Wall model-free methods and compared with CNCs prepared in sulfuric acid. On the basis of changes of apparent activation energy E-a a plausible mechanism of stabilization was discussed. The presented results showed the technological potential of organic acid anhydride application for manufacturing and thermal stabilization of CNCs.