Longest path in DAG or finding DAG diameter
But which method could help us to find the maximum path between all two vertices and pick the maximum one as graph Diam.


That's not how the diameter is usually defined; it's rather the maximum distance, i.e. the length of the longest shortest path. As such, any general-purpose APSP algorithm helps you find the diameter of DAGs.


  here we have a DAG so there is no cycle with Positive edge weight to drop in infinite loop for finding longest path.


Infinite loops are not the problem; finding longest paths is NP-hard in general graphs.


  Is any polynomial time order algorithm for this problem....?


As explained on Wikipedia [1], the Longest Path problem can indeed be solved efficiently on DAGs by finding the shortest path in the graph obtained by multiplying all weights by $-1$. 




Algorithms by R. Sedgewick and K. Wayne (2011)