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This present study aims to investigate development of morphological structures, which focuses on lexical morpheme (i.e. past –ed marking), phrasal morpheme (i.e. plural –s marking on nouns in contexts with quantifiers) and inter-phrasal morpheme (i.e. 3sg –s marking on verbs with the 3rd person singular subject in the present tense), in written English as L2 based on Processability Theory. The data was in the form of written essays produced by two university students learning English in an instructional context and was collected at three points in time during the period of eight months. Based on the data, a distributional analysis of individual subjects was conducted. Then, the findings were analysed by using the implicational scaling according to the emergence criterion to determine the acquisition points of the target structures in individual subjects. The results show that the developmental stages of the morphological structures under scrutiny of the two subjects (i.e. Learner 1 and Learner 2) generally follow the PT predictions – that is, lexical morpheme emerges before phrasal morpheme and phrasal morpheme emerges before inter-phrasal morpheme. Furthermore, the predictions of PT are followed in the written production of English as L2, which indicates its capacity to account for development of L2 morphological structures in both written and oral language production.