As for the field of educational research and its applications, evaluating concept difficulty is necessary but difficult to carry out the work. Two major approaches are employed in previous research to evaluate the difficulty of concepts. Both of two approaches do not take into account whether concepts are acquired by learners and readers or not. This paper will be focused on constructing basic concept list of domain knowledge with latent semantic analysis (LSA), and use the age of acquisition of concept for representing the difficulty of concept. This paper will utilize natural science texts of elementary school in Taiwan as experimental materials to verify the validity of using our proposed method for evaluating the difficulty of concept.