Disclosed is a method to produce cellulose nanomaterial from red algae marine biomass Gelidium amansii. The method includes the step of isolating cellulose from Gelidium amansii red algae marine biomass. The method includes the step of treating the red algae fiber in three stages such as alkalization, bleaching treatment, and acid hydrolysis treatment. The isolated cellulose nanomaterial has an average diameter of 21.8 - 11.1 nm with the average length of 547.3-23.7 nm and corresponding to an average aspect ratio of about 25-4. The obtained cellulose nanomaterial from the present method includes a high crystallinity index of 73% with high thermal stability at Tmax value of 330-334-degree centigrade.