Both Heidegger and Gadamer emphasize the importance of ”questioning” in the process of thinking. Therefore what thinking is can be revealed in the analysis of ”questioning.” However regarding ”questioning,” profound differences are shown in the philosophy of Heidegger and Gadamer. By analyzing the differences in their concepts of ”questioning,” the differences between their concepts of thinking can also be revealed. Based on this analysis, we can attain the conclusion that regarding the concept of thinking, Heidegger's theory cannot be justified; on the contrary, Gadamer's theory is well founded.