Effects of feeding dairy cows with grass hay, corn silage or corn silage with formic acid as forage on milk yield and composition, some blood parameters, reproduction and calve health were observed in this study. Thirty Holstein cows at seven month pregnancy and in their second or third lactation were divided into 3 groups. Three liter/tone farmic acid (Kemisile) was added to one part of the corn harvested from the some land, other part of the corn contained no additives. Animals in dry period were fed at a restricted level but forage was offered ad libitum in the lactating period and the amount of concentrate feed was arranged according to their milk yield. Feed consumption, live weight gain, milk yield were recorded and the health conditions of all calves were carefully monitored. Blood samples were also collected from animals six times during the experiment to determine serum SGOT, SGPT and urea levels. It was found that formic acid incorporation into silage increased nutrient content of silage in the current study. Compared to cows fed with normal silage, cows fed with corn silage containing formic acid had higher milk yield and milk composition. Statistically significant differences were found between the groups (P<0.05). However, milk from cows fed with grass hay contained more fat than that of cows fed with corn silage hawing entire low cellulose content during the whole experiment. Live weight gain was higher in the group fed with normal silage and corn silage with formic acid than those fed grass with hay before calving (P<0.01), whereas live weight lost after calving in cows fed on silages was found to be higher compared to cows fed with grass hay (P<0.01). Effects of feeding cows with silages treated with formic acid on these parameters were insignificant. No differences were found between groups in terms of serum SGOT, SGPT, urea levels, reproduction and calves health (P>0.05). According to the results of this study, formic acid addition to silages in dairy cow rations has increased the silage quality, decreased nutrient loss in silages and had beneficial effects on milk yield and composition. Formic acid is recommended as a food additive to silages in dairy cow rations.