The mathematical representation ability is essential in solving mathematical problems, especially in algebraic expressions problems. Therefore, it is crucial to have a valid design of learning activities to support students' mathematical representation ability. Several previous studies claimed that realistic mathematics education is one of the learning approaches that could support this ability. This study is design research aiming to evaluate the RME-based learning trajectory oriented to enhance students' mathematical representation ability on algebraic expression. The data collected from the documentation of learning trajectories, documentation of students' answer sheets, and video recording of online teaching and reflection sessions were analyzed descriptively. The findings indicated that the designed RME-based learning trajectory (LT) oriented to support students' mathematical representation ability in algebraic expression has been valid and could be implemented in the pilot experiment. The implementation of the LT-1 and issues found during the pilot experiment are discussed in the paper. The finding implies that the learning trajectory could be continued to the teaching experiment phase after some revisions and adjustment.