The purpose of this study is twofold. Firstly, it attempts to determine the pre-service science teachers' misconceptions about floating and sinking. Secondly, it aims to reveal the level of pre-service science teachers' misconceptions, scientific knowledge, lack of knowledge, and lack of confidence related to floating and sinking. To conduct the study, a diagnostic instrument of sinking and floating (DISF) with three tiers was developed and used. The three tiers were used to calculate a KR-20 reliability coefficient of 0.804 for the scientific knowledge and a KR-20 reliability coefficient of 0.768 for the misconceptions. The data was collected from 377 pre-service science teachers from three different universities in Turkey. 74 misconceptions about floating and sinking were found. These misconceptions were classified under seven categories. The results of the study demonstrated that the pre-service science teachers had many misconceptions, low scientific knowledge level, and low confidence level in their knowledge about floating and sinking.