This study investigates the low-frequency noise characteristics of the ${p}$ -type organic thin-film transistors (OTFTs) with dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT) by considering density-of-states (DOS). The OTFTs are fabricated with and without a self-assembled monolayer (SAM) on the SiO2 gate dielectrics. A significant reduction of 1/ ${f}$ is observed in the OTFTs with SAM compared to those without SAM. Moreover, this reduction is more pronounced in the high drain current region, indicating that the excess noise is reduced due to the lower DOS resulting from the introduction of SAM.