To investigate the changes of chitosan molecular weight, chitin and chitosan manufacture processings were carried out with and without acetone solution and nitrogen gas. Chitin yields were 22.5% (in T1) and 13.1% (in T4). Maximum viscosity of T2 was the highest value by 1,554 cPs at 3 h after reaction, but deacetylation was 44.3%. Chitosan viscosities of T1 and T2 were 414 and 738 cPs, respectively, at 5 h after reaction. At the same time chitosan viscosities of T3 and T4 were 150 and 318 cPs, respectively. Chitosan of T2 and T4 treated with nitrogen showed higher viscosity than chitosan of T1 and T3 without nitrogen supply. In test II, the viscosity of chitosan and reaction time in 40% NaOH treatment showed a low value with increasing temperature, reaching a maximum of around 70% deacetylation. The expected optimal condition was 846 cPs and 71.9% at 90°C for 5.9 h with 40% NaOH. However, in practice, the optimal condition was 786 cPs and 70.3% at 110°C for 4 h with 40% NaOH