Reliability is one of the most important topics researched in Systems Engineering. Due to the peculiarity of satellites and their operating environment, it is hard to use probability or statistic methods for the study of reliability engineering of satellites. To find reasonable methods to improve reliability of satellites is essential in aerospace applications. There are several kinds of method to improve reliability of satellites, robust parameter design and operating windows methods are two of them. One of a new method for improving in-orbit satellites＇ reliability, Fault Mode Avoidance （FMA）, is proposed and applied to improve reliability of satellites in this paper. One-Side and Two- Side failure modes of satellites, such as prediction of status space of failure mode, separation of failure mode, etc., are identified, and mathematic definitions and avoidance procedures of the failure modes are given. Several failure-mode-avoidance strategies are designed, and relative cases of the strategies are also shown.