This research explored the effectiveness of using the word network technique to teach English vocabulary to first graders in Thai Primary schools. The 150 top-frequency words used in three nationally-used textbooks were inventoried. The list of the words then was compared with the list proposed by the Ministry of Education, Thailand. The combined words based on semantic categories found in two sources of wordlists were then derived to form word networks as the models for the participating teachers. To carry out the study, both quantitative and qualitative research designs were used. In choosing the representative sample of the participants, 48 teachers who taught the English subject for first grade level and 206 first graders in Songkhla Primary Educational Service Area Office 1 participated voluntarily in this study. Most of the teachers had no degree in English or any related fields, but they were assigned to teach English courses. The research instruments were the students' scores from the pre-test and post-test, the word network diagrams, classroom observations, and the teachers' semi-structured interview. Results showed that there were significant differences between the students' mean scores of the pre-test and post-test. The participating teachers were able to adapt the ideas of using word network technique to create the diagrams to teach vocabulary and related structure to their students after training.