This study has endeavored to unearth the relationship between Iranian students' motivation, demotivation and their speaking skill. Additionally, the relationship between their L2 motivational self-system and the improvement of their speaking skill. And, finally, the relationship between their motivation/demotivation and their L2 motivational self-system. Considering the available literature, the internal structure of language motivation has been greatly debated and widely researched. Nevertheless, regarding the breadth and depth of the issue, we, inevitably, conclude that a lot has remained to be fully dealt with. Moreover, motivation, as an underlying construct can be tangibly manifested in a productive skill like speaking. To set out the study, 100 intermediate students who were also IELTS candidates in Nami language institute, Isfahan, Iran were selected through convenience sampling. The study was conducted in descriptive and experimental phases. Qualitative research procedures were applied for data collection by administering certain questionnaires and tasks which are delineated in the body of thesis. Then they were completed by subsequent quantification and interpretation of the coded data. The findings revealed that the relationship between students' demotivation and their speaking skill is negative but significant; in other words, the more demotivated the students, the lower scores they received. Regarding their L2 motivational self-system, the study showed that students' L2 self-system had a significant effect on their speaking skill. The study also found a reciprocal relationship between students' motivation/demotivation and their L2 motivational self-system. All the findings will be elaborated in the body of the thesis. Keywords: Speaking skill/Motivation/Demotivation/L2 Motivational Self-System