Flexible side chains have not drawn much attention in the development of organic semiconductors compared to the conjugated backbone counterparts. In this work, a series of BN-substituted tetrathienonaphthalenes (BN-TTNs) with methyl to hexyl side chains were synthesized to systematically investigate the influence of alkyl chain length on the solid-state properties and transistor performance. The intrinsic electronic properties of the π-conjugated backbone were not affected by different alkyl chains, but the solid-state properties, such as molecular packing structures, energy levels, thin-film morphologies, and transistor performance, were significantly influenced. Among the six compounds, BN-TTN-C3 exhibited the highest hole mobility of 0.15 cm2 V−1 s−1, whereas BN-TTN-C2 and BN-TTN-C4 did not show any field-effect mobility. This unprecedented difference of device performance was mainly caused by different thin-film morphologies. An odd–even effect of alkyl side chains on the thin-film morphology was observed for the first time, which further greatly influenced the device performance. This pronounced influence of alkyl chain length on the device performance indicates that alkyl chains play a vital role in organic electronics and should be paid more attention in future development of organic semiconductors.