In current pandemic era, all aspects of learning must be conducted online, including assessment. Quizizz is regarded as one of digital assessment tools or electronic classroom response systems that can be used effectively for formative assessment. The study aims at examining the online formative assessment processes used by Quizizz to practice mock English tests, as well as ascertaining students' perceptions of their learning experience with the Quizizz application. The data were collected from 97 English majored students at a Hanoi University of Industry (4 classes) during a period of home-based learning due to the COVID19 pandemic in the 1st semester, academic year 2021-2022. A study with mixed methods was used to achieve these goals. The data were collected using survey questionnaires and semi-structured interviews. The study uncovers two significant findings. First, it was confirmed that using Quizizz for formative assessment successfully increased students' performance during the teaching-learning process. Second, questionnaire and interview responses showed that students positively viewed Quizizz as an effective and engaging online formative assessment tool for checking their knowledge of the subjects. Almost all students praised Quizizz's feedback feature for promoting self-correction and self-regulated learning. This study demonstrates that Quizizz is beneficial not only for academic purposes but also for motivational purposes, as it fosters an engaging, competitive learning environment. As result, the findings of this study will suggest teachers and educators in implementing this effective method of formative assessment in EFL classrooms. Additionally, the findings may be applicable to the development of an online educational tool to facilitate students' distance learning during COVID-19 lockdowns.