A hydrocarbon sensor based on stimulus-responsive polydiacetylene(PDA)-embedded polydimethylsiloxane (PDMS) microbead has been developed. The colorimetric hydrocarbon sensor was fabricated by employing a microfluidic method and displayed hydrocarbon-dependent colorimetric response, allowing rapid differentiation among saturated aliphatic alkanes. The PDA-PDMS composite bead sensor was found to undergo a faster swelling in a hydrocarbon of shorter alkyl chains. Confocal fluorescence microscopic analysis demonstrated that the colorimetric transition of the embedded PDA supramolecules initiated from the surface of the bead and propagated into the core of the bead. The hydrocarbon-dependent colorimetric sensor also allowed a facile detection of adulterated diesel oils. Thus, the sensor beads in a tube were found to grow faster upon contact with kerosene oil than with diesel oil owing to the different hydrocarbon chain length. By using this method, we were able to detect adulterated diesel oils containing 30% and 60% of kerosene oils.