Trade liberalisation which has been taken place around the world in recent years brings about a number of changes to implementing countries. Thailand is now committing itself to the liberalisation under ASEAN Economic Community (AEC), due to complete in 2015. Once the AEC takes effect, the labour markets across the region will be open for member countries' workers. To be able to reap benefits from liberalised labour market, workers need to overcome their language shortcomings and the government needs to provide facilities necessary for their proficiency improvement. To accommodate English improvement programme, therefore, this study aims at analysing the determinants of labour’s English language proficiency in Thailand. It focuses on the case of vocational students in specific schools in Bangkok area. In order to undertake this research, various statistical tests for variance and mean heterogeneity, and stepwise regression associated with OLS to estimate the predictive power are employed and discussed. The study finds that students’ grade records, attending extra curriculum English classes, and plan of working abroad are statistically significant in explaining Thai vocational students' English language proficiency in all skills. Other determining variables are found statistically significant in some English skills. This study's findings should offer insightful information for both private school entrepreneurs and government agencies in providing appropriate training for workers and prospective workers.