Phenology, growth stages and growth indices and their effects on yield and yield components of five cultivars and landraces of Iranian onion were studied in Neishabour Agricultural Research Station during 2005 and 2006. The experiment was in split plot based on randomized complete block design with three replications. Three planting dates 15 April, 30 April and 15 May were considered as main plots and five long-day onions Ghermez Azarshahr, Dorcheh Isfahan, Ishaqabad, Sefid Qum and Sefid Neishobour as sub plots. Growth indices were calculated based on accumulative growth degree day (GDD) up to the end of growth, and leaf area by the formula A= -93 + 1.83L+ 36.8C25. All onion cultivars which were planted in the first date, reached to the maximum leaf area index (LAT) at about 2500 GDD (22 August) and the equation LAI= e (a + bH1 + CH2) was the best prospected one which could explain leaf area changes of onion cultivars in different planting dates during growth period. Total dry matter (TDM) changes during growth period achieved from the equation DM= e (a + bH1 + CH2 + CH3) for calculation of dry matter index (DMI). This equation showed a R2>91% for all cultivars. DMI increment procedure in all planting dates and treatments was in sigmoid form and there was no difference among cultivars. The bulb yield was affected by planting date and the maximum yield was obtained from the first planting date (56.523 tha-1). Yield increase can be related to LAI increment, DMI and relative growth rate (RGR) of cultivars in the first planting date. Dorcheh Isfahan cultivar, affecting by planting dates, produced the highest yield.