Daily rainfall data for twenty five years (1980 – 2004) of Balasore (23059/ N latitude and 86016/ E longitude), located in the ‘North-eastern Coastal Plain’ agro-climatic zone of Orissa, were critically examined for establishing the long term averages of monthly rainfall during the monsoon season and its temporal variability. The average monsoon rainfall (monthly total) of 1315.2 mm was distributed in the proportion of 21.3, 23.1, 24.1, 19.2 and 12.3% from June through October, respectively. The variabilities in monthly rainfall during crucial months of June (47%) and October (80%) were relatively higher than in the remaining monsoon months. The probabilities of both the normal onset (23rd standard meteorological week) and withdrawal (41^st standard meteorological week) of monsoon rain were 48%. The probabilities of aberrations in seasonal (June to October) amount of rainfall was 64.8% with higher proportion of below normal (38.3%) than its above normal rainfall (26.0%) during June to October. At 25% and 50% probabilities, a stable quantum of rainfall was observed during 23^rd – 41^st and 25^th – 37^th standard meteorological week respectively.