This study aims to propose a practical theoretical framework in constructing a prototype lesson of online-mediated lessons in English for Domestic Tourism using social constructivism theory in investigating the quality of an online prototype lesson in terms of the validity, reliability and effectiveness and exploring students' opinions towards this online lesson. The sample group consisted of 20 students, and the research instruments were InConMedt, a pre- and post-test of WBST-EFT with a rating scale and the interview. The results from priori-validation indicated that this online prototype task posed high content and construct validity and reliability. The statistical results from dependent-samples of the t-test showed a significant improvement at the .01 level in the high- proficiency group. Most importantly, the two proficiency groups had positive opinions of this online lesson.