We report the detection of H13CN, HCN and upper limits of HC15N in Neptune's atmosphere with our ALMA observations. These measurements will allow us to constrain the 12C/13C and 14N/15N Isotopic ratio in Neptune's stratosphere. Our observations -performed on April 30, 2016- used the ALMA interferometer to search in Neptune's atmosphere for HCN(4-3), H13CN(4-3) and HC15N(4-3) rotational lines at 354.505, 345.339, and 344.200 GHz, respectively. These measurements were obtained using about 41 antenna of the 12m array, despite the angular resolution of 0.6'' and Neptune's angular surface diameter was 2.24'', only disk-averaged measurements allowed to detect with sufficient signal-to-noise ratio the H13CN line, while only upper limits of HC15N were obtained. We will present the analysis of these observations, which will include i) the vertical profile of HCN, ii) the isotopic ratio of 12C/13C and 14N/15N and finally discuss the origin of HCN in Neptune's stratosphere.