Quantitative risk assessment of pathogenic bacteria, protozoa, and virus contaminated in the surface water in the flood area of the central Thailand has been employed by simulation technique from upstream of eastern source of pipe water Pichit, Nakornsawan, Singburi, Angthong, Ayuthaya and Patumthani and that of western source of pipe water Kanchanaburi, Nakornpathom and Nonthaburi including pipe water in all 15 Bangkok water distributors between June 2012 and February 2013. Sample sizes of surface and pipe water were 150 and 290, respectively. The probability distribution of exposures of surface water and pipe water were Normal as Normal(25, 1 ml/person/day) and Normal(751.79, 679 ml/person/day), respectively. The concentration of pathogens was converted from their corresponding prevalence using Maximum likelihood technique. Sample volumes for bacteria and protozoa including virus were 500 ml and 2,000 ml, respectively. The exposure dose of Salmonella spp. of surface and pipe water in terms of range was 0.02-86 person-1year-1, those of Shigella spp., pathogenic E. coli, Vibrio cholera and Vibrio parahaemolyticus were 0.02-0.9 person-1year-1 and those of Entamoeba histolytica, Cryptosporidium parvum, Giardia lamblia and Norovirus were 0.001-4.7 person-1year-1, 0.001-0.06 person-1year-1, 0.001-7.6 person-1year-1 and 0.001-0.02 person-1year-1, respectively. The prevalence of pathogens with uncertainty in the form of Beta distribution including concentration and exposure dose were altogether used to calculate probability of exposure in the exposure assessment step and followed by probability of illness by using dose-response model in the hazard characterization step. The integration of these 2 probabilities became the risk estimates by using software to simulate up to 10,000 iterations. The risk estimates in the form of range at 95th percentile of Salmonella spp., Shigella spp., pathogenic E.coli, V.cholera and V.parahaemolyticus were 0.006-18,919, 0.00012-0.3, 0.0000004-0.0009, 0.0008-2.2 and 9-1,500 cases/100,000/year, respectively and those of Entamoeba histolytica, C. parvum, G. lamblia and Norovirus were 0.00001-2,796, 0.000002-0.03, 0.000001-692 &' 0.001-3.2 cases/100,000/year, respectively.