This study has two aims. The first one is to reveal the science teacher candidates' misconceptions about the density by using a four-tier diagnostic test, and the second one is to determine to what extent science teacher candidates have scientific knowledge, lack of knowledge, and misconceptions about the density. In order to achieve these aims, a four-tier density diagnostic test was developed with the KR-20 reliability coefficient of 0.753 based on the correct answers, and the KR-20 reliability coefficient of 0.528 based on the misconceptions. To determine the misconceptions about the density with the developed test, data were collected from a total of 470 science teacher candidates from seven different universities in Turkey. With the developed four-tier diagnostic test, 48 different misconceptions were revealed. The results of the research showed that about one fourth of science teacher candidates had a misconception about the density and that the ratio of six of the resulting misconceptions was over 10%. It may be suggested that this study conducted on science teacher candidates should also be conducted on middle and high school students and experimental studies should be carried out to eliminate the misconceptions that emerged in this study.