We report high-performance and air-stable organic field-effect transistors (OFETs) fabricated with thermally cross-linked bottlebrush polymers as gate dielectrics. By synthesizing bottlebrush polymers with a varying degree of polymerization, we investigate the effect of the density of alkyne functional groups on the on-set cross-linking time (t c) and dielectric properties of the bottlebrush polymer thin films. The bottlebrush polymer with the highest density of alkyne groups shows the lowest t c of 0.6 min, and t c values increase to 0.8, 1, and 30 min as the density of alkyne groups decreases. These t c values are unambiguously shorter than that (t c > 120 min) of the most widely used polymer gate dielectric, poly­(methyl methacrylate) (PMMA). The top-gate/bottom-contact OFET fabricated with poly­{N,N′-bis­(2-octyldodecyl)-naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl-alt-5,5′-(2,2′-bithiophene)} and the bottlebrush polymer gate dielectric cross-linked in 1 min shows the highest charge carrier mobility (μ = 0.16 cm2 V–1 s–1) and extended device lifetime (120 h) compared to those of OFETs fabricated with the other bottlebrush polymers and PMMA gate dielectrics. Considering the sufficiently reduced t c (≤1 min) and excellent device properties of bottlebrush polymers, these results demonstrate that thermally cross-linkable bottlebrush polymers without any cross-linking agents would be promising materials for developing high-performance and reliable OFETs fabricated by a high-throughput fabrication method.