This paper proposes a novel trimming method of thin-film resistors dedicated to the power modules. Thin-film resistors are utilized for applications including snubber circuits which avoids unwanted ringing appearing in the output voltage of power transistors. Our previous works have revealed that the applicable voltage of a NiCr thin-film resistor is thermally limited, and then the indispensable trimming process affects the degree of temperature rise in use. In this paper, we first formulate the relationship between the target resistance value and the trim dimensions using machine learning. With the obtained equation, we propose a new trimming method, which enables the trimmed pattern to reduce the variability of the maximum temperature rise. The experimental results show that the proposed trimming method can suppress the estimated range of the maximum temperature from 619K to 179K.