Zn diffusions in InP have been studied by electrochemical capacitance voltage. The InP layer was grown by metal organic chemical vapor deposition, and $Zn_3P_2$ thin film was deposited on the epitaxial substrates. The samples annealed in a rapid thermal annealing. It is demonstrated that surface hole concentration as high as $1 imes10^{19} extrm{cm}^{-3}$ can be achieved. When the Zn diffusion was carried at $550^{circ}C$ and 5-20 min., the diffusion depth of hole concentration moves from 1.51$mu extrm{m}$ to 3.23 $mu extrm{m}$, and the diffusion coeffcient of Zn is $5.4 imes10^{-11} extrm{cm}^2$/sec. After activation, the concentration is two orders higher than that of untreated sample at 0.30 $mu extrm{m}$ depth. As the annealing time is increase, the hole concentration remains almost constant, except deep depth. It means that excess Zn interstitials exist in the doped region is rapidly diffusion into the undoped region and convert into substitutional When the thickness of $SiO_2$ thin film is above 1,000$AA$, the hole concentration becomes stable distribution.