Here we present a novel vapor-annealing technique to improve morphology of pure-iodide perovskite (CH3NH3PbI3) thin-films and get pin-hole-free coverage with > 10μm grain size. X-Ray diffraction (XRD) measurement confirms that the annealed films are phase pure perovskite. The larger grain size leads to a significant increase in the effective recombination lifetime of charge carriers, with the best films showing lifetime of >50 μs. Solar cells fabricated with the large grain perovskite thin film shows significant improvement in short-circuit current density (Jsc) and open-circuit voltage (Voc).