Abstract: Chilling threshold and growth degree day (GDD) of two main pistachio pistillate cultivars were determined. Layout was factorial based on a complete randomized design with three factors, two cultivars (Qazvini and Ouhedi), 5 thermal levels (+2, 0, -2, -4 and -6 ^oC) and three developmental stages including dormant bud, swelling bud and fully bloomed flowers for chilling studies. Critical temperature for reversible tissue colour change was determined as -4^oC at bud stage, -2^oC at blooming bud and +2^oC at flower. Decreasing temperature down to two more degrees (e.g. -6 ^oC at bud) could shift the damage into the irreversible browning injury. For GDD measurements, three factors, including cultivar, thermal accumulation (calculation based on +4.5^oC) and phenological stages were considered. Kernel filling period varied in two cultivars; Ouhedi's bigger kernel required more time to grow fully and more growth degrees day. Qazvini needed 2561.044 GDD and 138.5 days for total bearing period (flowering to harvest), and 623.363 GDD and 30 days for kernel filling period. Ouhedi needed 2917.823 GDD and 160 days for total period, and 730.61 GDD and 33.5 days for kernel filling.