The digestion of diet and utilization of nutrients by sheep depends on rumen microbial fermentative digestion. Reproduction and growth rate of microbes is mainly determined by the availability of nutrient and energy levels in rumen. Therefore, rumen environment affects the extent of microbial protein (MCP) synthesis. The aim of this study is to evaluate the effects of licking the molasses-urea feed block on the rumen of sheep, including microbial biomass and fermentative efficiency. Eight sheep were selected and divided into 2 groups (control group and treatment group). The treatment group animals were supplied with molasses-urea feed block for ad libitum consumption. Rumen fluid was collected every 2 h and rumen fermentation parameters were measured. In addition, the population of majority bacteria were investigated by real-time PCR. The results showed that the population of majority bacteria is increased in rumen of treatment group animals (P < 0.05). The trend of daily change of rumen bacteria was same in all animals. Each bacterium quantity decreased gradually after feeding, and reached the lowest level after 2 h intake. It then increased slowly and reached the highest level after intake for 8 h. Finally, each bacterium quantity was back to same level as before intake. In contrast, the protozoa number raised to the highest in 4 h feeding and declined gradually, the concentration of protozoa in treatment group sheep was significantly higher than that of control group (P < 0.05). The pH of rumen liquids were in normal range and were not different between both groups. However, pH decreased from the highest level before feeding to the lowest level within 4 h. It was increased after intake for 8 h. The concentration of NH₃-N and MCP synthesis in the rumen liquids were both significantly higher than that of control group (P < 0.05), the highest concentration of NH₃-N and MCP were reached after feeding by 2 h and 4 h respectively. Block containing urea and molasses can be well utilized by sheep, it is conducive for rumen fermentation by the microbes in ruminant.