A liquid tunable grating in a micro-optical fluidic system (MOFS) chip is demonstrated in this paper. The on-chip grating is realized through a microfluidic control system comprising of multilayer laminar flow. The liquid grating provides tuning possibility through the modulation of refractive index (RI) and grating pitch. The liquid tunable grating can be employed for accurate measurement of refractive index of biological and chemical fluids (e.g. Phosphate Buffered Saline (PBS)) and therefore has enormous potential applications in Lab-on-a-chip based bioanalysis.