Resistance of microencapsulated fish oil (MFO) in the rumen and its effects on fermentability of nutrients in the rumen was studied in 3 experiments. The key parameter in these studies was the amount of oil released from the microcapsules and their effects on gas production and rumen degradability in comparison with unprotected fish oil (FO). In the first experiment the effect of rumen and abomasum pH on disintegration of MFO and amount of fish oil released was investigated. In the second experiment effects of MFO on in vitro gas production and rumen fermentation kinetics was undertaken and in the third experiment in vitro Batch culture was used in order to estimate the effect of MFO on rumen degradability. Experimental treatments in the first experiment were 1) simple Microcapsules 2) tannic acid treated Microcapsules and 3) Ca2+ treated Microcapsules. In the second experiment or gas production study 2 kinds of basal diet including the forage diet (dry alfalfa) and the complete diet (forage to concentrate with ratio of 60:40) with 8 treatments for each experimental diet: including; 1) the control (without supplement), 2) 4% fish oil, 3) 12% whey protein concentrate and 4% fish oil, 4) 12% whey protein concentrate and 1.2% tannic acid, 5) 12% whey protein concentrate, 4% fish oil and 1.2% tannic acid, 6) 16% simple microcapsules 7) 17.2% tannic acid treated Microcapsules and 8) 17.2% Ca2+treated microcapsules were used. In the third experiment dry forage used as basal diet with 6 experimental treatments including; 1) the control (without supplement), 2) 4% fish oil, 3) 12% whey protein concentrate and 4% fish oil, 4) 16% simple microcapsules 5) 17.2% tannic acid treated microcapsules and 6) 17.2% Ca2+ treated microcapsules with 24h incubation time. In the first experiment the amount of oil released of microcapsules in rumen were 74, 7 and 12% and in abomasum were 74, 59 and 67% for simple, tannic acid and Ca2+ treated microcapsules respectively. Resistance of tannic acid treated microcapsules in the rumen and abomasum was significantly better than other treatments. In gas production experiment for both diets, the tannic acid led to significantly higher gas production in comparison with other treatments. In batch culture in vitro experiment oil the amount of oil released from microcapsules were 78.72, 13.52 and 26.2% in simple, tannic acid and Ca2+treated microcapsules respectively. DM degradability was not affected by fish oil in tannic acid treated microcapsules (P>0.05). DM degradability was significantly higher in tannic acid and Ca2+ treated microcapsules in comparison with the unprotected fish oil. The results showed that microencapsulation of fish oil with the suitable ingredients should led to better performance in case of animal performances. It can be concluded that the desirable dairy products in from of fatty acids are achievable by microcapsulation method similar to what was undertake in these experiments.