We present the rest-frame optical spectrum of a strongly lensed galaxy at redshift z =1.7 behind the cluster Abell 1689. We detect the temperature sensitive auroral line [O III] 4363, which allows the first direct metallicity measurement for galaxies at z > 1. Our high signal-to-noise spectrum indicates that the target is an extremely low metallicity star-forming galaxy.We estimate an intrinsic absolute B band magnitude of M_{B}=-18.3 pm 0.1$, with a stellar mass of 4.4$pm1.2times10^{8}$ M$_{odot}$. This galaxy extends the luminosity-metallicity relation of star-forming galaxies at z > 2 by more than an order of magnitude. Given the double-nuclei like morphology and velocity profile of ha, we tentatively suggest that it could be a merger or a proto-rotating disk galaxy.
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