The blue swimming crab (Portunus pelagicus) is an important commercial species in Phuket Province, Thailand but it is difficult to handle these spiny crabs to measure their carapace length and width directly. On the other hand it is easy to measure their weight only as much handling is not needed in this case. The aim of this study was to make correlations between body wet weight and carapace length, as well as body wet weight and carapace width of blue swimming crabs from Andaman Sea, Phuket Province. We predicted that there should be positive significant correlations between body wet weight and carapace length, as well as body wet weight and carapace width. A total of 68 crab samples ranging from 8.38 to 12.20 cm in width, 3.60 to 5.90 cm in length and 41.55 to 95.38 g in weight were collected from Andaman Sea, Phuket Province and analyzed. In addition, these crabs were divided into two different size groups based on their carapace width (small size group: 8.38-9.91cm and large size group: 10.00- 12.20 cm). Highly positive significant correlations between crab body wet weight and body size (carapace length and width) were observed in case of both size groups. Now, it will be possible to calculate carapace length and width of blue swimming crabs from Phuket Province without measuring their carapace length/width but only weighing them.