It has been widely reported that even advanced level second language learners have problems with grammatical morphemes. Recently, especially in the minimalist framework, a number of studies have focused on the sources of this problem and discussed whether they are at the syntax level or not. In order to contribute to this discussion, we collected neuro-physiological data (the event related potentials) of Japanese learners' sensitivity to ungrammaticality caused by the lack of subject-verb agreement in their L2 English. Our data showed that Japanese learners showed sensitivity to ungrammaticality of the given sentence (P600) when subjects and verbs did not agree in terms of person (e.g., *I answers your letter) and that P600 did not appear when subjects and verbs did not agree in terms of number, even when 3rd person singular-was overused (e.g., *The teachers answers our questions; *Sam and Adam answers our questions). Based on this finding, it will be suggested that the difficulty of the correct use of morphemes is not solely attributed to mapping from syntax tomorphology; Japanese learners are likely to have problems with taking features in the numeration; and the asymmetry between person and number is attributable to the difference (s) of these two features : the former is inherent but the latter is optional; and/or the former is available in learners' first language but the latter is not.