A new derivative of cellulose was synthesized by reaction of the polymer dissolved in dimethylacetamide (DMAc)/5% LiCl and methacryloyl chloride. The control of the molar ratio between the reagents and the temperature permits one to obtain a specific degree of substitution (DS). Cellulose methacrylate (CEMA) is a semirigid polymer with a persistence length which decreases as the DS increases. Consequently, the polymer concentration Cp at which the anisotropic phase becomes stable gets higher as the DS increases. CEMA solutions gellify when irradiated by UV, due to the crosslinking reaction of lateral double bonds. Swelling in DMAc/5% LiCl and water depends on the CEMA concentration and irradiation time. The first appearance of optical anisotropy may be observed at a polymer critical concentration which is always lower than the Cb of the uncrosslinked polymer.