Recent studies on morphological processing have shown that morphologically complex words such as class II affixed words or regular inflected words are processed by decomposition whereas class I affixed words and irregular inflected words are processed as whole-word units. The present study aims at exploring whether such patterns found for native speakers of English emerge differently in accordance with L2 proficiency. First, the results attained for intermediate proficiency learners show that complex words benefit from more processing efficiency than simplex words, derivational words cost more difficulty processing than inflected words for high-frequency condition, and class II suffixed words suffer heavier processing burden than class I suffixed words. Second, the results for advanced learner group show that simplex words are processed more efficiently than complex words, inflectional words are processed with more difficulty than derivational words in low-frequency words, and class II suffixed words pay more processing cost than class I suffixed words in high-frequency condition. These findings suggest that L2 processing mode for complex words is contingent on proficiency level and surface frequency is mediated in the processing. Furthermore, it is implied that though the extent of dual-route processing is not as strong as that of target language users, L2 learners seem to be under progress towards dual-route processing as the growth of proficiency.