This study reports the result of an experiment which measures the effect of extensive reading-only on vocabulary acquisition of Vietnamese English as a foreign language (EFL) primary students. A group of 50 Vietnamese EFL primary students (pre A1-A2 level learners), aged from 7 to 11 years old, took part in a 30-day-extensive reading (ER) programme. The researchers selected a list of target words from the reading texts. To measure their vocabulary acquisition prior to and after the programme, a meaning recognition test of target words was delivered one week before the reading (pretests), meaning recognition, form recognition and form recall tests were conducted after the reading (immediate posttests), and one week after the reading (delayed posttests). We expect to gain insights into the extent of vocabulary acquisition of the students participating in the ER programme and explore the effect of ER on vocabulary acquisition.