To study weed flora and determination of richness, species similarity and diversity index of weed communities in canola (Brassica napus) fields of Golestan province, different cities were surveyed during 2017. Fields numbers were determined based on the planting area of canola, randomly. Randomized sampling method as W shape was conducted using 0.5×0.5 meter quadrate systematically. Then weeds population were identified and counted in flowering stage of canola. In this study, 55 plant species from 22 families were recorded. The most number of weeds were observed in Poaceae (10 species), Brassicaceae (6 species) and Fabaceae (6 species) families, respectively. Among different weed families, Poaceae, Brassicaceae and Polygonaceae had the most relative abundance in canola fields, respectively. The results showed that 81.81 % of recorded weeds were annual, 14.54% were perennial and 3.63 % were biennial. Also 76.36 % of identified weeds were broad-leaved and the rest were narrow-leaved. Winter wildoat (Avena ludoviciana) was the most important weed from narrow-leaved weeds and Black mustard (Brassica nigra) was the most important weed from broad-leaved weeds. The highest number of species was observed in Bandargaz (28) and the lowest was in Aliabad (7). Among different cities, Kordkoy and Gorgan showed the highest similarity index (86%). Galikesh and Aliabad cities had the lowest similarity index (33%). The highest Shanon diversity index was observed in Ramian city (2.68) and the lowest value was observed in Bandargaz city (0.55). The highest Simpson diversity index was observed in Aliabad (0.39) and the lowest value (0.15) was observed in Gorgan (0.0002).