The Space Robotics Laboratory (SRL) of Tohoku University has developed three 50kg Micro-satellites. The first satellite 'SPRITE-SAT' has been successfully launched into Earth orbit, and also been operated. The flight model of the second satellite 'RISING-2' has been assembled and its software development is now finalized, being ready for the launch planned in next year. The third satellite 'RISESAT' project is during the EM (Engineering Model) development phase at the time of writing. The launch is planned in the later half of 2013. The structural design of all these satellites is based on central pillar configuration, while continuous improvements have been made through the projects. The paper will discuss these aspects of structural design and evaluation summarizing the results of numerical analyses and mechanical tests conducted by the SRL during the past 5 years of micro-satellite development activities.