Abstract: A method of determination of microcystin in surface water by HPLC with solid phase extraction (SPE) was proposed.The water sample was first filtered under reduced pressure and microcystin in the filtrate was then enriched by passing through a C18-reversed-phase SPE column,which was eluted with a mixed solution of CH3OH+H2O (mixed in the ratio of 20+80 by volume),to remove substances coexisting with microcystin.The SPE column was dried by blowing with N2-gas,and microcystin was eluted from the column with pure CH3OH at a flow-rate of 1 mL·min-1.The eluate was collected and evaporated to 1 mL and used for HPLC determination.The Cosmosil C18 chromatographic column was used as the stationary phase and a mixed solution of 0.05 mol·L-1 phosphate buffer solution and methanol,mixed in the ratio of 40 to 60 by volume,was used as mobile phase.UV-detection at 238 nm was adopted in the determination.Under these conditions,2 isomers of microcystin,i.e.,MC-LR and MC-RR,were well separated,and linear relationship between values of peak area and concentration of the 2 isomers was obtained in the range of 0.25-10.0 mg·L-1,with determination limit (10S/N) of 0.05 μg·L-1.Values of recovery found by adding mixed standard solution of the 2 isomers to a blank water sample,were in the range of 76.2%-96.5%.