In Iran, the availability of high quality forages is restricted. The use of wheat straw, which is accessible in a huge amount, is limited due to its low nutritional value. Recently, biological method using white-rot fungi to improve the nutritional value of straw have been reported (Zadrazil 1997). However, the data available have been obtained from in vitro works. Additionally, little information is available on the utilization of fungal treated straw fed to dairy cow. Therefore, this experiment was carried out to assess the effect of fungal treated wheat straw on the performance of lactating cows.