Media multitasking behavior is extremely popular among Chinese adolescents. Such behavior has been found to be related to low cognitive control, low wellbeing, and poor academic performance. In a study of subjects engaged in two simultaneous media activities, particular effects associated with heavy multitasking included worse performance on attention, working memory, and switching tasks (indicating poor cognitive control). However, without a comprehensive and widely accepted definition of media multitasking, it is hard to make comparisons among different studies or run longitudinal investigations. This study defined media multitasking along three dimensions: (i) multitasking between media and media activities; (ii) multitasking between media and non-media activities; (iii) concertation without multitasking. That definition was used to develop and validate a new media multitasking scale for Chinese adolescents, using data gathered from a sample of 1140 such subjects.