This study was conducted during the growing seasons of 1998-1999 and 2003-2004 to determine the critical period of weed control (CPWC) in winter-lentil (cv. Sazak-91). The experiments were laid out in a randomised block design with four replications. The beginning and end of CPWC were based on 5% acceptable yield loss levels which were determined by fitting logistic and Gompertz equations to relative yield data, representing increasing duration of weed-interference and weed-free period, estimated as growing degree days (GDD). The results indicated that CPWC for seed yield was between 237 and 846 GDD in the first year and between 123 and 414 GDD in the second year, while CPWC for biomass was between 216 and 820 GDD in the first year and between 212 and 374 GDD in the second year. Thus weeds should be controlled from the first week after the onset of regrowing stage of the crop in spring up to 7th week for winter-lentil to avoid losses above 5%.