The linguistic competence of Thai university students majoring in Business English was examined in the context of knowledge of English language inflection, and also various linguistic elements. Errors analysis was applied to the results of the testing. Levels of errors in inflection, tense and linguistic elements were shown to be significantly high for all noun, verb and adjective inflections. Findings suggest that students do not gain linguistic competence in their use of English language inflection, because of interlanguage interference. Implications for curriculum reform and treatment of errors in the classroom are discussed.
 R. Ellis, (1994). The Study of Second Language Acquisition. Hong Kong: Oxford University Press, p. 15.
 C. Codrad, (2001). Tripod succeed online, Lycos Inc. Retrieved January 5, 2010, from Second language acquisition: http://chris1066.tripod.com/~chris1066/home.html.
 S. P. Corder, (1967). The significance of learners’ errors. International Review of Applied Linguistics, pp.5, 161-170.
 K. Hakuta, & H. Cancino, (1977). Trends in Second Language. Trends Haruard Educational Review, p. 297.
 D. B. Brown, (1994). Principles of Language Learning and Teaching. New Jersey: Prentice Hall Regents. pp 203 – 204.
 W. O’Grady, & J. Archibald, (2001). Contemporary Linguistics. New York: Bedford/St.Martin's.
 N. Stageberg, (1970). An Introductory English Grammar. Iowa: Holt, Rinehart and Winston Inc.