Current translation studies do not present a clear distinction between ‘translationese’ and ‘interlanguage’, giving rise to conceptual and terminology confusion. To disentangle these two concepts, we start with a relatively conservative working definition of translationese, then find it necessary to first differentiate between direct and inverse translations, according to whether the translator's L1 equals to TL or not. Taking Zhuangzi (a Daoist classic) as a case, we made both inter- and intra-speaker comparisons among Lin Yu-tang's inverse translation, James Legge's direct translation, and the two translators' creative works in English, with well-established language complexity metrics and quantitative methods. Results show that: (1) Inverse and direct translations are remarkably different in terms of complexity; (2) Inverse translation demonstrates both features of interlanguage and translationese, with the former mostly at lexical level and the latter at syntactic level; (3) Similar patterns are also discovered in Lin's other inverse translated works, suggesting our quantitative comparative method proposed may be reliable to some extent. Such results support our proposal that translationese and interlanguage should and can be differentiated for both theoretical and practical purposes.