To prevent the food poisoning originated by consumption of shellfish contaminated with domoic acid, the quantitative analysis of domoic acid is to be very important. The stability of domoic acid at different temperature, pH and light was investigated using high performance liquid chromatography (HPLC). The mean recoveries of domoic acid in the methanol extracts from oyster (Crassostrea gigas), blue mussel (Mytilus edulis), short neck clam (Ruditapes philippinarum) and ark shell (Scapharca broughtonii) were 85.4-104.5%, 94.8-101.2%, 91.0-104.6%, and 95.7-109.6%, respectively. The working solutions of domoic acid standard were very stable for one month at $-18^{circ}C$, $4^{circ}C$, and room temperature. And domoic acid in the methanol extract from oyster was stable for a day at $4^{circ}C$ and room temperature, and for a week at $-18^{circ}C$. Therefore, this implies that quantitative analysis for domoic acid must consider the storage conditions of the standard solutions and the methanol extracts from shellfish. The standard solutions adjusted to pH 3-9 were also stable after heating at $121^{circ}C$ for 30 min. The effect of light exposure on domoic acid was tested by exposing the methanol extracts to light. Domoic acid degraded slowly when the samples were kept in the dark (brown vial). However, following the light exposure the photodegradation became more rapid; no detectable domoic acid remained in $1.0{mu}g/mL$ of methanol extract after 5 hours.