The author proves that an integrable function f can be approximated by the Kantorovich type modification of the Szász-Mirakjan and Baskakov operators in the \(L^ 1\) metric in the optimal order \(\{n^{-1}\}\) if and only if \(\phi^ 2f'\) is of bounded variation where \(\phi (x)=x^{1/2}\) and \(\phi (x)=[x(1+x)]^{1/2},\) respectively.