Fluorescence resonance energy transfer-based technique is commonly used in analytical chemistry and bioimaging applications. There are various advantages of using fluorescent proteins as fluorophores for FRET applications. For example, the expression of protein level could be fine-tuned controlled temporally and spatially. The sequence, orientation, and length of linker could be rationally designed by protein engineering. This review summarizes the applications of FRET-based intracellular sensor.