Linear standard curves have been used to predict quantitative analysis in spectrophotometry. This research was conducted to find out how to determine the purity of biodiesel samples by FTIR methods using KBr and ATR-FTIR. Biodiesel synthesis was carried out by transesterification reaction at 65°C with a mole ratio of methanol: oil of 12:1 and a base catalyst of NaOH of 1% by weight of oil. The concentration of the biodiesel product was analyzed by 1H-NMR, which was used as standard biodiesel. Variations in the weight percentage of biodiesel to triglycerides were made of 30%, 40%, 50%, 60%, 70%, and 82% then FTIR analysis was carried out using KBr and ATR-FTIR methods. The results showed that in predicting the percentage of biodiesel purity resulting from the reaction, both FTIR methods could be used. Based on the analysis of the results of transesterification of palm oil with FTIR spectra, it is known that the linearity of the curve in both FTIR methods using KBr and ATR shows different areas