Four Holstein cows were used in a 4 ${ imes}$ 4 Latin square experiment to study effects of fat sources on rumen metabolism and digestibility of nutrients. All cows were fed a total mixed diets containing 60% silage and 40% concentrate. The four concentrates were formulated to contain either Megalac(MEG), formaldehyde-treated whole linseed(LIN), a mixture (50 : 50, oil basis) of fish oil and formaldehyde-treated whole linseed(MIX), or no fat source in the concentrate but 500g per day of linseed oil being infused into the duodemm (OIL). The rumen pH was lowest in OIL among the treatments(P < 0.05), but ammonia N concentration in the rumen was not significantly different among the treatments. The differences of total VFA, acetate, propionate, iso-butyrate and iso-valerate concentrations were not significant among the treatments. While, butyrate and valerate were highest in OIL and lowest in MEG(P < 0.05 and P <0.01, respectively). In addition, A:P ratio was also highest in OIL and lowest in MEG(P < 0.05). As expected, intake of nutrients(DM, OM, NDF and ADF) was lowest in OIL among the treatments(P < 0.01). However, all nutrients flow to the duodenum, and digestion in the rumen and total tract were not significantly different among the treatments. Intake of N was highest in MEG, but lowest in OIL treatment(P < 0.01). Duodenal flow of total N, nonammonia N and microbial N was not significantly different across the treatments. In addition, microbial synthesis and ammonia N and total N digestibility were not affected by different dietary fat sources. The present results show that fermentative characteristic and microbial efficiency in the rumen, and nutrients digestibility in the rumen and total tract were not depressed by supplementation of as much as 6% dietary fat sources.