Abstract: In conventional practice a row of separated circular jets is used in turbine blade or vane for impingement cooling and the relative distance z n/d n is very small ( z n/d n ≤2).In this case,study on longitudinal uniformity of the cooling on the surface of the blade or vane is necessary.In our experiments,the uniformity of the velocity distribution at large relative distance ( z n/d n >10) is achieved by free jet in atmosphere,but the distribution of jet velocity is rapidly uniformized when the jets srtike against the target surface and flow along the surface.The reason of the uniformization is the turbulence mixing and the effect of pressure difference.This work contributes to good understanding for uniformization of cooling on the surface of the blade or vane.