The present work examined the effect of different chitosan sizes (bulk and nano) on the shelf life of fillets made of Cromileptes altivelis flesh in cold temperatures. Research procedures involved the characterization of chitosan and nanochitosan from the prawn skin (the study used the Litopenaeus vannamei species) and the preservation of C. altivelis fillet in chitosan and nanochitosan solution during the cooling process. Fresh C. altivelis flesh weighing 800-950 gram were sliced into skin-off fillets before being immersed in chitosan and nanochitosan solution for 15 minutes. The flesh was then covered and stored at a cold temperature (± 5⁰C) for nine days with an observation interval of three days. Observations were made on the values of TVB-N, TMA-N, pH, TPC, FFA, peroxide value, and the sensory values of humpback grouper fillets. The present work employed a randomized block group with a factorial pattern. All parametric data were analyzed using the ANOVA and Duncan tests. The experiment was divided into three chitosan and nanochitosan treatments with a duration of 0 day, 3 days, 6 days, and 9 days. The results showed that chitosan and nanochitosan caused significant differences (P< 0.05) in the values of TVB-N and TPC. Similarly, a significant difference was recorded in all test parameters' storage time (P< 0.05). Using nanochitosan solution can suppress the degradation of fish fillets in terms of the TVB-N and TPC values. Moreover, the best treatment was the fillet soaked in nanochitosan solution that was stored for six days. The present work concluded that nanochitosan affects the shelf life of C. altivelis fillet during the cooling process.