The aim of this study was to examine prospective science teachers' knowledge of and difficulties with the metric unit conversion. The participants of the study were 73 prospective science teachers. In this qualitative study, a measurement test with 14 questions was administered to the participants to examine their knowledge of and difficulties with unit conversion. The questions of the test were related to metric measurement units for length, area, volume, and mass, as well as to the knowledge of the approximate size of a body and some uses of metric units. For the first 11 questions, participants' answers were evaluated as right or wrong. To examine the reasons lying behind their difficulties, their explanations on the past three open-ended questions were analyzed. The findings indicated that prospective teachers' performance on unit conversion was not satisfying in general, and their major difficulties were mainly related to the conversion from gram into microgram, mg into g, ml into cm[superscript 3], dm[superscript 3] into mm[superscript 3], gigameter into nanometer, mm[superscript 2] into m[superscript 2], and determining the relationship between centigram and dekagram. The implications for the field are discussed in light of the related literature.