In this work, on-chip bio/chemical sensor was reported based on localized surface plasmon resonance of nanocrescent patterns fabricated via electron beam lithography. The nanocrescent arrays with different dimensional features exhibited controllable plasmonic properties in accordance with the simulation results based on the finite-difference time-domain model. The highest refractive index sensitivity of the fabricated samples was achieved to be ~699.2. nm/RIU with a figure of merit of ~3.1 when the two opposite crescents own a gap of ~43.3. nm. Such obtained plasmonic sensor was further integrated into the microfluidic system which can simply control the specific analyte concentrations via tuning the flow rate ratios between two injecting microstreams. Our method has successfully demonstrated the capability of the nanocrescent patterns as on-chip plasmonic bio/chemical sensor for real-time monitoring of dynamic concentrations in the microchannel.