In present paper, we propose a new diagnostic test to measure students' conceptual knowledge of principles of modern physics topics. Over few decades since born of physics education research (PER), many diagnostic instruments that measure students' conceptual understanding of various topics in physics, the earliest tests developed in PER are Force Concept Inventory (FCI, Newtonian concepts), Force & Motion Conceptual Evaluation (FCME), Electric Circuits Conceptual Evaluation (ECCE), and Test of Understanding Graphs-Kinematics (TUG-K). Although these tests were generated and tested on the fields, they were mainly interested on freshman physics courses. Maybe only diagnostic test developed above freshman was the one initially used by researchers to investigate college students' understanding of quantum physics concepts but unfortunately, its source or history is not known. The main purpose of this study is to declare of a new diagnostic test and reveal initial results of the diagnostic test of Test of Understanding on Concepts of Modern Physics (TUCO-MP).