The performance of organic field-effect transistors (OFETs) with high-k cross-linked cyanoethylated pullulan (CLCEP) polymer as a gate dielectric was studied. The optimized film spin-coated from the polymer blend showed high dielectric constant (13.4), high capacitance (53.4 nF cm⁻², at 1 MHz), and negligible hysteresis enabling reliable and low-voltage OFET operations (−3V). With optimized cross-linking temperature, a field-effect mobility of 1.32 cm²V⁻¹ s⁻¹, an on/off current ratio (I_on/I_off) of ∼10⁵ and a threshold voltage (V_th) as low as −1.05 V were obtained. Also a low inverse subthreshold slope (SS) of 89 mV dec⁻¹ was obtained which is close to the theoretical value of 57 mV dec⁻¹ at room temperature. The number of trap states was estimated from threshold voltage shift and SS values, and was confirmed to be related to the OH intensities measured via FT-IR. The morphology and microstructure of the pentacene layer grown on CLCEP dielectrics were also investigated and correlated with OFET device performance.