Abstract: The castor bean seeds (Ricinus communis L.) are raw material for many industrial products and its importance has rise at developing countries in the past few years, since, castor seed oil can be used as substitute of petroleum for biodiesel, plastics and lubricants manufacture. The aim of this study was to observe the effects of a dose range of gamma radiation (0.5 to 10 kGy) over the physiological quality and seed-borne micloflora from two lots of Brazilian castor bean seeds. Seed vigour was assessed by a novel computer imaging analysis system and linear regression analyses were performed for the tests. Castor seed moisture content was not significantly affected by gamma radiation. First counting germination, germination and seedling emergence in sand were inhibited at doses = 2 kGy. The vigour index and seedling length were also severely reduced at higher doses. Both germination and vigour tests identified the lowest vigour of one of the seed lots. The incidence of some fungi as Alternaria alternata, A. ricini, Penicillium sp. and Aspergillus sp. was inhibited at doses = 5 kGy, but Fusarium sp. and Cladosporium sp. survived at exposures up to 8 kGy. Sterilization doses higher than 2-5 kGy would not be suitable as castor bean seeds revealed to be quite radiosensitive up to that dose range.