We derive a new relationship between the giant branch slope as measured in the color-magnitude diagram (K, J - K) and [Fe/H] metallicity for old open clusters. Previously such relationships have been derived for globular clusters, while similar tendency has been expected for open clusters. New derived correlation, [Fe/H]=-17.2($pm$0.23)GB slope - 1.95($pm$0.02), is based on a collection of data for 10 old open clusters. Most clusters behave as expected from the theoretical predictions.