Crude oil is a products of oil and gas industries. One of the parameters which has in crude oil is specific gravity. In the oil and gas industries, specific gravity is used to classify crude oil which to be produced. specific gravity can be measured using ASTM D-1298 method. Disadvantage of this method is there are need of special environments and still using manual transcription. This research has been conducted a crude oil specific gravity sensor design by implementing structural imperfect to improve the performance of specific gravity sensors. In this research, polymer optical fiber is used as a sensor to measure specific gravity of crude oil. There are three variation of the amount of defect that were used in this research. Sample which used in this research has span of specific gravity value from 0,842218 to 0,9097. Based on the result, sensor with 3 imperfect structure have the best performance with linearity coefficient value 0,9438 and the span of measurement is 1,75792 dBm.