This study aims to develop an online test that can be used to characterize students' conception of objects in static fluid. The test developed by carried out several stages, namely: modified previous essay test and formed the Tier-1 of multiple-choice questions with options taken from the findings previous studies; combined the reasons for choosing options in Tryout-1 and the findings of previous studies for forming Tier-2; expert validation process; refinement process, readability test tryout with revisions; data collection involving 318 pre-service elementary school teachers to characterize the online test based on item response theory; and revised some items. Rasch analysis was implemented to characterized the test and subjects. The developed test consists of 23 items in two-tier covering the context of floating, sinking, and suspending objects. The results of validation showed that in the development process, the test has very good for item validity. Rasch analysis showed that, overall, the test functioned well with its item reliability is very good category. Based on the findings and difficulty level, the test can be used for middle school students up to college level with some notion. Some limitations of the test are presented in this paper.