Abstract: Calcareous sand is a special kind of geotechnical sand which consists of much carbonate. Based on the remains of the original marine organisms, the calcareous sand grains have irregular shapes and contain inner pores. The inner pore structure has great effect on the mechanical properties of the calcareous sand. To investigate the distribution of inner pores, the calcareous sand samples from the South China Sea are divided into six groups according to grain size, and a series of scanning electron microscopy (SEM) tests are carried out on different shape grains from each group. MATLAB is used to process the images got from the SEM tests with proper threshold values. The effects of grain size and grain shape on inner pores of calcareous sands are analyzed. The results reveal that, for the sand grains with size less than 1 mm, the porosity increases with the growth of grain size and there is no obvious difference between the porosity of grains with different shapes. However, for the sand grains with size greater than 1 mm, the porosity decreases with the growth of grain size and the strip shape grains have the largest porosity while the slice shape grains have the smallest one.