In Thailand, raw milk used for human consumption should have somatic cell count in bulk tank (BSCC) less than 500,000 cells/ml. Although most BSCC of bulk milk from dairy cooperatives were less than 500,000 cells/ml, BSCC from smallholder dairy farms in cooperatives were sometimes higher than this limitation. Therefore, the objective of this study was to determine the proportion of farms with higher than 500,000 cells/ml and their associated factors. Data of BSCC from 3 dairy cooperatives in Chiang Mai and Chiang Rai provinces during Sep 2010 to April 2012 were used. For dependent variables, the BSCC data were recoded 0 and 1 for farm without and with BSCC>500,000 cells/ml, respectively. Independent variables included the available of veterinary service in the area and seasons, as cool (Nov to Feb), summer (Mar to May) and rainy (Jun to Oct). Factors associated with BSCC were determined by the significant association (p<0.05) between dependent and independent variables using repeated measure analysis by generalized linear mixed models. Based on the total of 189 smallholder dairy farms, results showed that the overall percentage of farms having BSCC>500,000 cells/ml were 24.6% (923/3,758). Percentages of farms having high BSCC in 3 cooperatives were 41.3, 21.0, and 21.3, respectively. Farms in the cooperative without the available of veterinary service had about 3 times more risk (OR=3.34) of having high BSCC (p<0.01). Proportions of having high BSCC were significantly associated with seasons. Farms in rainy had higher risk of having BSCC than in cool (OR=1.68) and summer (OR=1.83). In conclusion, the milk quality control based on BSCC need veterinary service and should be emphasized during rainy.