Link scheduling is crucial in improving the throughput in wireless networks and it has been widely studied under various interference models. In this paper, we study the link scheduling problem under physical interference model where all senders of the links transmit at a given power P and a link can transmit successfully if and only if the Signal-to-Interference-plus-Noise-Ratio (SINR) at the corresponding receiver is at least a certain threshold. The link scheduling problem is to find a maximum "independent set" (MIS) of links, i.e., the maximum number of links that can transmit successfully in one time-slot, given a set of input links. This problem has been shown to be NP-hard [10]. Here we propose the first link scheduling algorithm with a constant approximation ratio for arbitrary background noise N ≥ 0. When each link l has a weight w(l) > 0, we propose a method for weighted MIS with approximation ratio O(min(log, log max l∈L l min l∈L l )), where l is the Euclidean length of a link l.