Organic field-effect transistor (OFET) is a device that controls the current supplied to the connected other functional components such as organic light-emitting diode (OLED), organic photo-diode (OPD) or various sensors in organic electronic applications, and a fundamental unit that determines the extent of the organic electronic applications. The magnitude of the current flow from source to drain in the OFETs is proportional to the amount and the mobility of charges induced in the semiconducting organic films. Thus, the amount and the mobility of charges are of major concerns to consider for the further improvement of the performance of the OFETs. However, relatively few studies have investigated on the density of states (DOS) function that affects the amount of charge, while much research has been done on the mobility. Herein, the fundamental correlations between the DOS, charge carrier mobility and device performance in OFETs based on a series of the cyclopentadithiophene(CDT)-based Donor-Acceptor (D-A) type conjugated copolymers were investigated. The DOS of the CDT-based copolymers was extracted using the temperature-dependent experiments and the mobility measured from the corresponding OFETs was compared. Based on the results, the material structure and electrical property relationships of CDT-based D-A type copolymers will be discussed to provide the guidance for the rational design of high performance organic polymeric semiconducting materials.