Understanding variation in SW (SEED weigh) is of major importance in understanding SY (SEED yield) variation. The objective of this study was to determine factors such as SN (SEED number) per unit area, SFD (SEED filling duRATIOn) and SFR (SEED filling rate) and temperature, LAI (leaf area index), LAD (leaf area duRATIOn), above-ground dry matter, CGR (crop growth rate), leaf dry matter remobilization and efficiency and SSR (source-sink RATIO) around SFP (SEED filling period), affecting SW in canola (Brassica napus L.). The experiment was conducted at Agricultural Research Station of Gonbad, Iran, during 2005-7. Two cultivars of spring type canola (Hyola401 and RGS003) as subplots were grown at 5 sowing dates (SDs) as main plots, spaced approximately 30 days apart, to obtain a wide range of environmental conditions during SFP. The experiment was arranged in two conditions, i.e. supplemental irrigation (SI) and rainfed (RF). SW was influenced by the growing season rainfall and temperature. The availability of the crop to produce and to remobilize photosynthetic assimilates to developing SEEDs was a good determinant factor for SW. SW increased with increase in LAI, above-ground dry matter production and remobilization and SSR around SFP, leading to an increased SY, suggested that SW primarily depends on the resource availability. The relationships of SW with SFR and SFD and above-ground dry matter, LAI, leaf dry matter remobilization and efficiency and SSR around SFP, over environmental conditions, sowing dates and cultivars, showed these variables to be generally applicable in canola SW determination.