Sea lettuce(Monostrima nitidum) were treated with several alkali agents on condition with water blanching at 9$0^{circ}C$ for 10 min., and followed by drying and powdering, thus, the power samples treated and the not treated were then allowed to be stored at $25^{circ}C$, dark place for 5 months. The sea lettuce powder samples were evaluated for green color intensity and volatile flavor at a month intervals. The green intensity of the samples were measured by using Hunter-lab colorimeter, therefore, the decoloration of greenness were seen in both of the samples treated and the not treated, but there were slight inhibitory effects on decoloration in the treated with alkali agents, especially in the treated with KHCO$_3$.The volatile flavor of the samples were collected by simultaneous distillation-extraction, and then the 31 flavor compounds were separated on HP-5 capillary column(25m$ imes$0.25mm i.d) and identified by using GC-MS. From these results, it was presumed that the characteristic impact flavor compounds were $eta$-cyclocitral, $eta$-cyclohomocitral, ionene, $alpha$-ionone and $eta$-ionone. The total content of the characteristic impact flavor compounds decreased in the samples treated with alkali agents more alkali agents more than in the not treated, but lee decrease was observed in the treated with KHCO$_3$