A new practical technology for separately sensing of strain and temperature using a single fiber Bragg grating (FBG) is proposed and demonstrated. The FBG can be think to be a grating composed of two inner FBGs with the same period but different diameter when a part cladding diameter of the FBG is decreased. Theoretical analysis and experiment data show that the two inner FBGs have the same temperature response but different strain response. So the strain and temperature responses can be measured separately and the span between the two inner FBGs can be used to measure the temperature-independent strain. In experiment, a part of cladding diameter of a FBG is decreased to 82 μm by etching it in HF acid. The strain responses of two inner FBGs are 0.00201 nm/με and 0.000858 nm/με, respectively, the strain response of the span between two inner FBGs is 0.00116 nm/με, and the temperature responses of two inner FBGs both are 0.01 nm/℃. Based on these experiment data, the strain and temperature can be simultaneously measured using a single FBG.