This paper reports the frequency dependence deterioration of the ac resistance found in Litz wires for high power applications. Litz wire is widely utilized for heating coils of the induction heaters as a wire with small ac resistance at high frequency. Particularly, the recent spread of next-generation switching devices resulted in high-frequency operation of high-power induction heaters, which increasingly adopt Litz wire with a large diameter for copper loss reduction. However, little knowledge has been reported on the effect of the Litz wire diameter on the frequency dependence of the ac resistance, which will be important for the design of Litz wire for high power applications. To elucidate the effect, this paper experimentally measured and compared the frequency dependence of the ac resistance among the Litz wires with different diameters. The result revealed that the Litz wire with a large diameter exhibits worse frequency dependence of the ac resistance than that with a small diameter. In other words, the Litz wire with a large diameter increased ratio of the ac resistance to the dc resistance at a lower frequency than that with a small diameter. An analysis of this reason was also carried out using the recently proposed Litz wire copper loss model, which revealed that the copper loss by the internal proximity effect tends to prevail in the large-diameter Litz wire to cause deterioration of the frequency dependence of the ac resistance. The analysis also suggested that sparse distribution of the strands as well as thinner strands can be effective to mitigate this effect, which can be a key to solve this problem in the Litz wire for high power applications.