We report a cross-linkable diblock copolymer at a mild temperature by Diels-Alder click-chemistry for a gate dielectric layer in the top-gate, bottom contact organic field-effect transistors (OFETs). The Diels-Alder reaction between poly[(methyl methacrylate)-co-(9-anthracenyl methyl methacrylate)] (P(MMA-AMA)) and 1,1 '-(methylenedi-4,1-phenylene)bismaleimide (MPB) enables the preparation of robust cross-linked films at 100 degrees C annealing for 10min. Poly(9,9-dioctylfluorene-alt-bithiophene (F8T2) or poly(3-hexylthiophene) (P3HT) OFETs exhibited charge carrier mobilities of 3 similar to 4 x 10(-3) cm(2) V-1 s(-1) with the 250nm thick crosslinked (P(MMA-AMA)) films at gate voltages of less than -20V. Moreover, the ratio of MMA and AMA was controlled to observe the effects of crosslinking density on the properties of the gate dielectric layer. When the portion of a crosslinkable anthracenyl unit in (P(MMA-AMA)) is increased to 50mol.% in the base polymer, the gate leakage current of OFETs decreases below 10(-8) A at V-g = -20V.