In the context of Vietnam's international integration, there is an increasing demand for a labor force that can use English to communicate effectively in their work place. In order to meet this demand, many Vietnamese universities now offer English for Specific Purposes (ESP) courses. However, these ESP courses are often developed by teachers who have no professional training and little experience in ESP and the quality of these ESP materials is questionable. With a view to improving the quality of ESP materials, this paper analyzes some weaknesses of the book "English for Physics" which was used at the Faculty of English, Hanoi National University between 2004 and 2010. On the basis of this analysis, the question of what should be taken into account when designing ESP materials will be discussed and suggestions will be made. The three main suggestions for ESP material design are based on a student needs analysis, ESP material writing that follows a systematic procedure, and ESP material writers who have some principles in mind when they present a guide to the process of wnting. It is hoped that this study will give an insight into ESP materials that are used in Vietnamese universities and elsewhere.