This study aims to obtain more insight regarding the effect of multimedia learning on third grade of Thai primary pupils' achievement in Size and Depth Vocabulary of English. A quasi-experiment is applied using "one group pretest-posttest design" combined with "time series design," as well as data triangulation. The sample comprises 42 pupils of third grade of Municipal School 2 located at Hadyai, Songkla Province, Southern Thailand. The main analyses were carried out using "t-test" and "Anova." The findings confirm that the implementation of multimedia to delivering the learning material through an integrated mode (text, picture, and sound) is able to improve pupils' achievement in size of vocabulary knowledge but not to depth of vocabulary knowledge. The findings imply that the policy makers and English teachers in Thai, together, are required to innovate the current English learning system, including facility development, multimedia program development, teachers' role reorientation, and parent involvement. The originality of this study lies in that the more insight as regards vocabulary proficiency in the context of Thai primary pupils has been urged by several researchers. Both the result and research methodology applied in this study was perceived to be useful for further researches and applications.