While the dissipative effect on the orbital lifetime of LEO has been widely discussed, the effect from gravitational on low-orbit satellites is also important. For high altitude satellites, the gravitational perturbation due to the third body means that its orbital eccentricity will so increase that it will eventually fall down onto the primary. In this paper, we focus on the low-orbit satellites. Though the result is similar, the mechanism is totally diffeent: the lifetime of low orbiters depends on the ratio of oblateness to the zonal harmonic, while that of high orbiters depends on the attraction of the third body. A theoretical analysis and numerical examples are given here for cases in the solar system.