The purpose of the study was to theoretically develop a measurable instrument of an ideal L2 self with six sub-factors: the degree of desire, the vividness of mental imagery, learning strategy, the degree of plausibility, the frequency of thinking about the future, and the degree of fearfulness. All factors were supposed to be part of one of the categories of human emotion, cognition, expectation, and desire. Through the process of scale construction, the study confirmed that this instrument had statistically reliable and valid factorial structures. After an exploratory factor analysis and confirmatory factor analysis, ideal L2 self was able to justify the hierarchical sub-scales. The analysis of the structural equation modeling revealed the strong influence of ideal L2 self on the language learner’s effort, learning desire, and learning autonomy. Therefore, this instrument helps us systematically understand the concept of ideal L2 self and observe other factors related to ideal L2 self as well as effectively highlights the discrepancy between Gardner’s integrative motivation and the ideal L2 self.