Under the condition of protecting cultivation,the relevance among variability of SPAD values at different leaf positions and the nitrogen content of leaves in melon critical growth period was studied,the best leaf position of N diagnosis for muskmelon which based on SPAD meter was explored.The results showed that in different nitrogen situation,leaf SPAD value at different leaf positions were different.Leaf SPAD value raised with increasing nitrogen fertilization in all leaf positions.Under the optimum level of nitrogen(N 210 kg/hm2),the variation coefficient of top 3rd leaf and top 4th leaf SPAD value were significantly lower than those of the top 1st and 2nd leaf in each growth stage.Leaf SPAD value of different leaf position had good correlation with N content of leaf and plant.The top 3rd leaf had the best correlation with leaf and plant nitrogen content.Consequently,when the nitrogen nutrition of muskmelon was diagnosed by SPAD chlorophyll meter,the ideal indicator leaf was top 3rd leaf.