The citations to articles published in Applied Physical Letters (APL) and Journal of the American Ceramic Society (JACS) were analyzed. For each journal, 199 articles published in 1995 were randomly selected and the number of citations in each year, CY, to these articles during 1996 ~ 2004 were recorded. It was found that CY reaches its maximum in 1997 for APL and in 1999 for JACS. During the first three years after the year when the articles were published, the number of self-citations to the articles published in APL is about two times of that in JACS. These results seem to indicate that the impact factor, which is defined as the ratio of the number of citations to a journal to the number of articles published in this journal over a two-year period, is not comparable for different journals in different fields.