Abstract The engineering Test Satellite VI (ETS-VI) is a three-axis-stabilized satellite launched by NASDA in 1994. On-orbit experiments, which consist of identification and attitude-control experiments, were planned as one of the ETS-VI on-board experimental programs, and were carried out in 1995. This experimental program was called ‘FLEX’ (FLexible EXperiments). The purpose of FLEX was to establish fundamental attitude-controller design methods for a flexible-structure satellite. In this paper, attitude controller design methods for FLEX are discussed, and some results of the attitude-control experiments are presented. The attitude controllers were designed using an H ∞ control method and a two-degree-of-freedom control method. H ∞ controllers were derived using the specified stability degree method. On the other hand, conventional two-degree-of-freedom control design methods could not be applied for the FLEX, because the program memory area was restricted to about 2.6 Kbytes. In this paper, feedforward control design methods which never increase the order of the controller were proposed and applied. Experimental results showed a rapid tracking response and high disturbance-attenuation performance, which have been anticipated by simulations on the ground. Noting that the on-orbit experiments presented here have not previously been carried out, the authors believe these results will be utilized for attitude controller design in the future.