Demotivation is an issue, which has been recently the focus of attention in the field of second language (L2) learning and teaching [8] (Muhonen, 2004). Since the present researchers failed to find any studies conducted in this area in Iran, this study was carried out to investigate Iranian high school teachers' and students' ideas about demotivating factors with regard to practicing the speaking skill. To achieve this goal, 312 high school students and 92 high school teachers of English were interviewed. Then the interviews were thematically analysed. The findings along with the researchers' reviews of the related literature were used to develop a questionnaire to explore Iranian L2 students' and teachers' perceptions of the factors which decrease students' motivation to improve their L2 speaking ability. This questionnaire was administered to 150 Iranian male and 150 female EFL learners and 40 male and 40 female teachers. To analyse the data, the researchers employed principal components analysis with varimax rotation. The factors which emerged were negative attitude toward learning L2, teacher's inadequate competence and performance, lack of technological facilities in classroom, lack of adequate teaching materials, unfavourable classroom environment and insufficient opportunities for speaking practice. The interpretable factor structure, the high loadings of items on the abovementioned factors and the relatively high amount of variance accounted for by this factor structure show that the questionnaire is a valid instrument to be used in similar studies. Also, the internal consistency estimates of the factors show that they enjoy high reliability.