Needs analysis is the key step to designing relevant and useful courses in English for Specific Purposes (ESP), but finding ways to deliver needs in a motivating and interesting way is also paramount. Therefore, investigating students' motivation to learn ESP is central in needs analysis. Few studies have investigated needs from the perspective of ESP learners' motivation. Focussing on engineering fields at three Iranian state universities, this study aimed to identify factors that affect engineering students' motivation for learning technical English in ESP courses. Using a mixed method design, this study was conducted in two phases. In the first phase, semi-structured interviews with 40 undergraduate students helped to identify motivational variables, select a theoretical framework and develop instruments for the second phase. In the second phase, a survey using a 28-item questionnaire with 152 students, semi-structured interviews with 60 students, in-depth interviews with eight ESP practitioners and non-participant observation in seven ESP classes were done in parallel. Principal Component Analysis on survey data resulted in five motivational components of main sources of engineering students' motivation to learn technical English: (1) ideal-self, (2) ought-to self, (3) instrumentality promotion motivation, (4) learning conditions, and (5) learning experience. These components were in agreement with Dörnyei's L2 Motivational Self-system. Further quantitative and qualitative data analyses revealed that long-term goals and the practicalities of using technical English in every day life promote motivation more than a sense of obligation to learn technical English. Moreover, the results showed that Engineering students had negative attitudes towards elements of ESP learning experience, which best explained the reasons for students' low motivation. The findings also supported the significant effect of academic levels on motivation. The factors that motivated students call for modifications in ESP courses which are discussed as pedagogical implications.