The purpose of this study was to develop and test a new instrument, the Medical Intermediate Concept Measure of Ethical Reasoning (Hereafter the MD-ICM), designed to help educators assess the ethical reasoning of medical students in Thailand. Following earlier methods for developing an ICM measure (e.g., Bebeau & Thoma, 1999), the dissertation first presents the development of the MD-ICM . Specifically, there were four steps used to create the measure: Step 1: identify appropriate ethical dilemmas, Step2; construction of the Items used to assess choice and justification information, Step 3: obtaining expert evaluation of the ethical dilemmas and assessment items items. and Step 4: Testing for reliability and validity. Reliability and Validity of the measure was assessed in a sample of 627 Thai Medical Students representing seven different universities. These students were in the first three years of medical training. Quantitative analysis (e.g., descriptive statistics, simple correlation, ANOVA, and regression analysis) was used to answer four research questions. The findings from this study are; 1) the MD-ICM was able to distinguish medical students who differ in medical ethics instruction and experience in the medical profession. Superficially, third year students scored higher than second year and first year students respectively. Further females scored higher on the MD-ICM across years and schools. 2) MD-ICM scores differed between students experiencing two types of professional ethics instruction. Students experiencing a specific course in medical ethics scored higher than students from schools who have adopted an integrated program. 3MD-ICM scores relate to established measures of moral judgment development supporting the convergent validity of the measure. Further, the results of these analysis indicates that the MD-ICM offers information that is not redundant with these established measures., and 4) the relationship between MD-ICM scores and supporting measures did not vary by gender. Results are discussed in terms of the applicability of the MD-ICM approach to the assessment of professional ethics education.