To investigate the effect of WEED interference and WEED-free periods on Balangu (Lallemantia royleana Benth. in Wall) and to evaluate the critical period of WEED control (CPWC) in this oilseed crop, an experiment was conducted as a randomized complete block design with three replications in Naghadeh region in 2018. Experimental consisted of two groups of treatments; WEED infestation treatments (infestation for 144.2, 345.9, 574, 848.2, 1147.6 and 1451.3 growing degree day (GDD) after transplanting and then plots were remained WEED-free the rest of the growing season) and WEED-free treatments (WEEDing for 144.2, 345.9, 574, 848.2, 1147.6 and 1451.3 GDD after transplanting and then plots were remained infested until harvesting time). In each block, one WEED free and WEED infested control was for all the growing season. Results indicated that the density and dry biomass of WEEDs were increased as WEED removal was delayed. In contrast, density and dry biomass of WEEDs were decreased as WEEDs establishment was delayed. Height, tetrachenes fruits, ripened achenes, 1000-seed weight and biological, seed and relative yields of Balangu were decreased and increased as the duration of WEED-infested and WEED-free period increased, respectively. Also, logistic and Gompertz curves fitted to data showed that the beginning and end of the CPWC based on a 10% yield loss of relative yield, was estimated to be from 11 to 108 days after transplanting (97 days WEED-free period) in order to prevent a noticeable yield loss.