Perceptual assimilation is a well-known task; however, there is no study on the assimilation pattern of the English monophthongs by L2 Thai learners. The aims of this study are to explore the perceptual assimilation patterns of the British English monophthongs to Thai monophthongs by L2 Thai learners and to examine the effect of L2 experience on this perception. The target British English sounds were /i:, I, e, ae, [upside down a], a?, [backwards c], [near-close back rounded vowel], u:, [upside down v], 3;/ in /bVt/ context. The Thai listeners performed an assimilation task by matching these British English monophthongs with their L1 Thai monophthongs. The results showed no difference in the assimilation patterns between the high-experienced and low-experienced groups in the perception of the English /I, e, a:, [backwards c], [near-close back rounded vowel], u:, [upside down v], 3:/. The degree of the perceived similarity in the matching of these vowels to the Thai sound categories between these two groups was not significantly different from one another either. However, English /e/ was mostly perceived as Thai /e/ in the high-experienced group to a greater degree than the low-experienced group. The findings also showed the difference in the assimilation patterns between these two groups, i.e. for English ae, i:, u:, [upside down a]/ suggesting the importance of the L2 experience in the exploration of the L2 speech learning research. The implication for L2 sound learning of this study is that having higher number of phonemes in the L1 phonological system than that in the L2 one is less important than the L2 experience.