This study aims to develop a test that can be used to explore mental models and representation patterns of objects in liquid fluid. The test developed by adapting the Reeves's Development Model was carried out in several stages, namely: determining the orientation and test segments; initial survey; preparation of the initial draft; try out; expansion of coverage; expert validation; trial and proving validity-reliability; and test finalization. The content and construct validity were determined by involving two experts as raters. The test reliability was determined using Cohen's Kappa (?), which involved 62 students and combined the test-retest and inter-rater methods to determine the consistency of students' responses. The developed test consists of 30 items covering the context of floating, sinking, and hanging objects. The hanging objects are dominant context of the test. The results showed that in the development process, the test has very good validity, and high reliability. The test can be used to explore mental models and external representation patterns on students in elementary, junior, senior high schools and undergraduate. Furthermore, limitations of the test are also presented in this paper.