The water quality, phytoplankton and coliform bacterial distribution in the reservoir of Ratchamangkla Park, Chiang Mai province were exmined for five months during August to December 1997. It was found that the water quality assessed on the tropic levels was at mesotrophic status. The water level was 5.53-6.28 m, Secchi depth 1.27-1.7 m, tuebidity 3.0-5.0 NTU, water temperature 25.5-30.4 ° C, pH 6.82-8.36, alkalinity 1.53-1.61 meq.l^-1, conductivity 145.3-181.2 m s.cm^-1, dissolved oxygen (DO) 5.6-7.6 mg.l^-1, biochemical oxygen demand(BOD)0.3-1.7 mg.l^-1, nitrate-nitrogen 0.5-1.0 mg.l^-1,ammomium-nitrogen 0.01-0.12 mg.l^-1, soluble reactive phosphorus (SRP) 0.01-0.12 mg.l^-1, chlorophyll a 8.43-9.72 m g.l^-1. The coliform bacteria and faecal coliform bacteria (MPN.100 ml^-1) determined by multiple tube method were 23-240 and 4-240 respectively. According to classification by the standard surface water quality of Thailand, the reservoir was in the second category, for house hold consumption when properly treated. The investigation of species and quantity of phytoplankton in this reservoir indicated 6 divisions, 14 orders, 33 families, 73 genera and 130 species. The dominant species was Cylindrospermopsis raciborskii (Wolosz.) Seenayya & Subba in the Division Cyanophyta, followed by Planktolyngbya limnetica Lemm. In the Division Cyanophyta and Monoraphidium contorium (Ther.) Kom-Legn. In the Division Cyanophyta. The species and quantity of phytoplankton in the reservior tended to show the quality of water as mesotrophoc status.