This study aimed to investigate the production of stress in English compound nouns and phrasal verbs by L1 Thai learners. Based on the Interlanguage Hypothesis (Corder, 1982; Selinker, 1972), it was hypothesized that (1) there is a positive relationship between English proficiency levels and accuracy in stress assignment in compound nouns and phrasal verbs; and (2) L1 Thai learners' systematicity of L2 English stress placement is influenced by L1 transfer. The participants were 60 first-year undergraduates who were equally divided into two groups, namely intermediate and advanced groups, based on their English proficiency levels. All the participants were required to read sentences containing three different types of compound nouns in the first task and read sentences containing compound nouns and their corresponding phrasal verbs in the second task. Their readings were analyzed using an independent-samples t-test and ANOVA. Although the advanced learners outperformed their intermediate counterpNaarts in assigning stress in both tasks, the statistical results indicated a correlation between English proficiency levels and accuracy in stress placement only in compound nouns, but not in phrasal verbs. It was assumed that such systematicity found in the learners' IL resulted from the interlanguage factor of language transfer.