The population dynamics of the green shield scale, Pulvinaria psidii (Mask.) (Hemiptera - Coccidae) was studied for two successive years (2008-2009) on guava trees at Shibin El-Qanater district, Qalubiya Governorate. The obtained results revealed that, P. psidii occurred on guava trees all the year round and has two overlapping generations a year. The 1st generation started from early March to early August/mid-August, peaked in mid-May (early summer) with duration of 5.0 - 5.5 months at field conditions of 20.7 - 21.3°C and 70.7 - 71.9%R.H. The 2nd generation occurred from early May to mid-November, peaked in mid-August (late summer) with duration of 6.0 - 6.5 months at 24.2 - 25.0°C and 69.4 - 70.4%R.H., respectively. The favorable time for abundance of P. psidii occurred in early and late summer during both high temperature and relative humidity. The adult population was relatively higher than nymphal population one in winter months and this may be due to the cold weather and most of the nymphs attained to the adult stage which sheltered on stems bark or in the stem cracks. Daily mean temperature and %R.H. were effective on both nymph and adult populations in 1st and 2nd generations in the two studied years, the population was correlated with the increase of temperature. The combined effect of the daily mean temperature and %R.H. on both nymph and adult populations was high on the both generations. The changes in the half monthly counts of nymph and adult populations referred to the effect of the tested weather factors on the 1st generation ranged 83.8 - 87.3% & 55.8 - 75.9% and 55.7 - 69.1% & 51.4 - 56.6% for the 2nd generation in the two studied years, respectively.