Anxiety in speaking English is a critical affective reaction to second language acquisition. Moreover, language learning is an emotionally dynamic process which produces fluctuations in learners' speaking anxiety. Therefore, this case study was designed to investigate English as a foreign language (EFL) learners' speaking anxiety from an ecological perspective based on nested ecosystems model and complex dynamic system theory. Four intermediate level female students with an average age of 15 were selected and participated in this study. Data were collected via semi-structured interviews recorded by the researchers over five classroom sessions, non-participant classroom observation and Motometers to provide information regarding the dynamics of students' anxiety during these 5 sessions. The data were qualitatively content analyzed. Based on (Bronfenbrenner, The ecology of human development, 1979; Bronfenbrenner, The ecology of cognitive development: Research models and fugitive findings, 1993) nested ecosystems model, the emergence of learners' speaking anxiety were categorized and analyzed first at the level of microsystem in terms of learners' beliefs, motivation, cognitive factors, linguistic factors, affective factors, and classroom environment. Afterwards, the participants' anxiety within three ecosystems including meso-, exo-, and macrosystems were also discussed as they were offered by the collected data. Learners' anxiety was also analyzed based on the dynamic patterns of stability and variation in the participants' micro development. The findings contributed evidence to the ecological understanding of the patterns and variables involved in learners' speaking anxiety variation in light of the interaction of the individual and environmental factors.