Numerical analysis was performed to investigate the behavior of rammed aggregate piers (RAP) in soft ground with various interface conditions, area replacement ratio, aspect ratio and surcharge loads of pile and soil. And field modulus load test was carried out to predict the input parameters. Field prototype (unit cell) tests are in progress to compare the result of numerical analysis. Also a modified load transfer equation of RAP on soft foundation was proposed. According to the results, the behavior of RAP depended on such as interface conditions, settlement characteristics (free strain) and stress concentration ratio. On the other hand, maximun stress concentration ratio increased as area replacement ratio and aspect ratio increased, and it was remarkably affected by surcharge loads.