Light bridge (LB) is bright structure crossing the umbra of sunspots and associated to the breakup or assembly of sunspots. In this paper, a LB is presented and studied using the observatory data obtained by {it Hinode} satellites. Force-free factor ($alpha$) and the z-component of current ($J_{z}$) and tension force ($T_{z}$) are calculated basing on the vector magnetograms observed by Spectro-Polarimeter (SP) of the Solar Optical Telescope (SOT) on board {it Hinode}. It is found that the amplitudes of $alpha$ and $J_{z}$ of LB are generally larger than those of umbra. It is found that there are two signs of $J_{z}$ along LB, which are divided at near the middle position of LB. It is found that the amplitudes of $T_{z}$ of LB are smaller than those of umbra and there are changes of sign of $T_{z}$ between the boundary of LB and umbra. Through comparisons and investigations, it suggest that LB and umbra maybe two different magnetic systems, which is a necessary condition for interaction magnetic reconnection.
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