Wireless ad hoc networks are expected to provide broadband services parallel to their wired counterparts in near future. To address this need, MIMO (multiple-input-multiple-output) antenna techniques hold significant promise. Most previous work on capacity analysis of ad hoc networks is based on an implicit assumption that each link exclusively occupies a geometric area, referred to as exclusion region that characterizes the amount of spatial resource occupied by a link. When multiple antennas are deployed at each node, however, multiple links can transmit in the vicinity of each other simultaneously, as interference can now be suppressed by spatial signal processing. As such, the concept of "exclusion region" no longer applies. In this paper, we investigate link-layer throughput capacity of MIMO ad-hoc networks. In contrast to previous work, the amount of spatial resource occupied by each link is characterized by the actual interference it imposes on other links. To calculate the link-layer capacity, we first derive the probability distribution of post-detection SINR (signal to interference and noise ratio) at a receiver. The result is then used to calculate the number of active links and the corresponding data rates that can be sustained within an area. Our analysis shows that there exists an optimal active-link density that maximizes the link-layer throughput capacity. This will serve as a guideline for the design of medium access protocols for MIMO ad-hoc networks. To the best of knowledge, this paper is the first attempt to characterize the capacity of MIMO ad-hoc networks by considering the actual PHY-layer signal and interference model. The results in this paper pave the way for further study on network-layer transport capacity of ad-hoc networks with MIMO.