Composition of raw milk impact on the price of milk produced and satisfaction of consumer. Thus, the objective of this study was to investigate the variation and association of milk compositions. The total 40,511 milk composition records were collected from 432 dairy farms located in Central part of Thailand between January 2012 and December 2014. Milk compositions included milk fat (Fat; %), solid not fat (SNF; %) and total solid (TS; %) in bulk milk tank. Dairy farm size was classified into 3 groups as small size (1 to 9 milking cows), medium size (10 to 19 milking cows), and large size (20 to 90 milking cows). Year-season at determination and dairy farm size were considered as fixed effects, and farm and residual were used as random effects in mixed linear model. Variance ratio and least squares means were reported using Mixed procedure and association of milk compositions using Correlation procedure. Results showed that difference in year – season has influence on Fat, SNF and TS (P < 0.001) but non-significance for farm size (P > 0.01). The difference of composition in milk produced by dairy farmer was associated with difference among farms for 28% to 34% because different farm management under production conditions of individual farm. The correlation between TS with Fat (r = 0.88; P