The permeability of calcareous sand has significant influence on the formation of underground freshwater in coral island. Since the contents and occurrence states of fine particles play important roles on the permeability of calcareous sand

it is necessary to analyze their effects and relationships. Permeability tests with constant hydraulic head were carried out on different fine particle contents and particle size combinations of the calcareous sand collected from an island in South China Sea. Results show that the upper bound of particle size affecting the permeability of calcareous sand is 0.075 mm. When the minimum particle size is less than 0.075 mm

the permeability coefficient of calcareous sand is 10?2 cm/s and can be defined as medium permeability according to the permeability coefficient. When the minimum particles size is between 0.075 mm and 0.500 mm

the permeability coefficient is 10?1 cm/s

which shows high permeability. The stable permeability coefficients of the calcareous sand change with the increase of fine particle contents

and the relationship shows as the following: 1) When the fine particles contents are less than 9%

the permeability coefficients decrease slowly with the increase of the fine particles contents. 2) When the fine particle contents are within the range of 9%-24%

the permeability coefficients decrease rapidly with the increase of the fine particles contents. 3) The permeability coefficients tend to be in steady state when the fine particles contents are more than 24%. The fine particle content that affects the permeability coefficient is determined by the pores formed by the skeleton

which reflects the limit value of the fine particle content when the pores are optimally filled. Poor packing or excessive fine particles can result in the migration and loss of fine particles under infiltrate process.