To study the weed population indexes of onion (Allium cepa L.) monoculture systems in the Jiroft area, four onion continuous monocultures, including less 3 (S1), 3-5 (S2), 5-8 (S3), more than 8 (S4) years and long-term fallow (F) were sampled by systematic randomized method in three stages: before sowing, 60 days after planting and 30 days after harvest, during 2015-2016. Of the 28 weed species, 16 species were observed in all systems. The analysis of variance showed that the monoculture system had a significant effect on the density of the weed species including Cyperus rotundus, Malva neglecta, Heliotropium europium and Chenopodium murale and the density of these species were was 14.64, 14.16, 13.90 and 11.97 weeds per square meter, respectively. Increasing the in monoculture duration from three to more than eight years led to enhancing increased the density of the weed species. The assessment of the onion monoculture duration had a significant effect on the weed diversity indexes. The maximum indexes of Simpson, Shannon, Menhinick and evenness in the S4 system were 0.92, 2.76, 0.90 and 0.99, respectively, and they were the lowest in the F system (0.78, 0.89, 0.78, and 0.95, respectively). In general, the onion continuation cropping increased the weed density of each weed and significantly enhanced the species diversity indexes.