Abstract: This study investigated autonomous English learning implemented by EFL students at an upper-secondary school concerning the utilization of online resources and the perceived affordances and challenges. To be specific, this study aimed to find out the frequency of upper-secondary EFL students using online resources for their language learning out of the classroom and the constraints that the students encountered when conducting their online autonomous learning. Quantitative research design was employed in the current study with the use of questionnaire. The participants of the study were 158 students at Le Quy Don High School for Gifted Students in Quang Tri Province, Vietnam. The findings revealed the prevalence of such online resources as online dictionaries, videos on YouTube, and social media. Conversely, radio, discussion forums, and emailing received the least preference. The most frequently-reported affordances related to stable Internet connection, whereas the most frequently-reported constraint was financial resources. From the findings, implications for teaching and learning with online resources and suggestions for further research were put forward.