This article investigates the effect of application of theme-based model of Content-Based Instruction (CBI) on a group of science students learning English at a preparatory program at the tertiary level in Turkey. In this study, course materials based upon the theme-based model of CBI were applied in order to teach academic English language skills. With a purpose to determine the language needs in the program, questionnaires including structured interview questions were distributed to a number of 97 students of Physics, Chemistry and Biology departments and also 19 students who specifically studied thematic units completed a questionnaire as to the effect of such an application. The majority of students reported that they needed to learn English for academic reasons in order to pursue future courses taught in English. Those students who studied science related thematic units argued that their motivation to learn English increased as they were able to improve their academic language skills through content they received and the activities they were involved in. Data findings may also indicate that if the theme-based model of CBI is applied in the preparatory program widely, it can meet language needs since such a practice involves learners in science English, equipping them with necessary academic language skills.