In order to investigate diversity of weed species in wheat fields in eastern parts of Mashhad, an investigation was made in the year 2009. Wheat fields was classified based on the acreage from 3-5 ha, 5-10 ha and higher than 10 ha. Sampling was made randomly by a quadrate 0.5×0.5 in 9 fields of each group from jointing to heading of wheat in tow separate stages. Type and total number of weeds were recorded in each quadrate. The results showed that total number of weed species in first sampling were 20 from 10 different plant family and in second sampling 18 species from 8 plant families. In both sampling, number of dicotyledons species (30) were more than monocotyledons (8). The highest diversity in dicotyledon was observed for compositae and cruciferae with 7 and 9 species respectively. For monocotyledons, gramineae family with 8 species showed the highest diversity. There was a negative relationship between the size of fields and shanon index. The highest shanon index in the first and the second sampling was observed (1.33 , 1.85) respectively with 2 ha area and the lowest shanon index in the first sampling was observed with 30 and 20 ha (H´= 0) in the second sampling, these value were 0.46 and 0.5 for 30 and 20 ha respectively. In general, species richness and similarity index were low in the studied area.