We demonstrate an ultra-high range of threshold voltage modulation in pentacene transistors by iodine doping. Through diffusion control and thermal treatments, the transistor's threshold voltage can be tuned in a wide range of more than 100 V from enhance mode to deep depletion mode. The correlation between physical and electrical characteristics was investigated to elucidate the modulation mechanism. The results suggest an optimistic potential for pentacene transistors in high voltage device applications. The transistors in this research exhibit a high field effect mobility of similar to 0.18 cm(2)/Vs and on/off ratio similar to 10(5).