To determine the threshold of photoperiodism and culmulative thermal time related to bulbing and growth analysis of Safid Behbehan, Ghermez Iranshahr and Primavera onion cultivars an experiment was conducted at Behbahan Agricultural Research Station for a year (2006-2007) based on a randomized complete block design of four replications. Seeds were sown in nursery in late October and seedlings transplanted in late January. Earliest time of bulbing was assessed using the bulbing ratio (maximum bulb diameter divided by minimum neck diameter). Bulbing occurred for Ghermez Iranshahr, Safid Behbahan and Primavera cultivars at a photoperiodism of 12 h and 7 min (19 March), 12 h and 52 min (15 April) and 12 h and 34 min (4 April) and cumulative thermal time of 1036.3, 1317.2 and 1179.8 growth degree days respectively. Bulbs were harvested when 50-80% of the foliage tops had fallen and started to collapse. The results of growth analysis indicated that the period of slow growth had been 135 days after emergence (for all the studied cultivars) and then the period of rapid growth of plants being started. Leaf area index (LAI), relative growth rate (RGR), crop growth rate (CGR) and bulb growth rate (BGR) of Safid Behbahan cultivar was higher than those in the other cultivars, so that this cultivar produced the highest yield of bulb dry weight. The yield of Bulb fresh weight in Primavera cultivar was significantly (1% level) higher than that in the other cultivars. However, due to the observation of high significant difference among bulb dry matter percentage in the studied cultivars, analysis of variance was done on yield of bulb dry weight. The obtained results indicated that the yield of bulb dry weight per hectare of Safid Behbahan cultivar was significantly (5% leval) higher than that in the other cultivars.