5-Ethylthio-1H-tetrazole quality (purity) which is one of the most important active intermediate, as an activator for condensation in solid-phase synthesis of oligonucleotide was controlled.Thin-layer chromatog. (TLC) and ion pair reversed-phase high performance liquid chromatog. (IP-HPLC) were studied in this article.To establish the system of dichloromethane-methanol-acetone-triethylamine (6:1:1:0.2) on silica gel GF254 plate, perform a HPLC method on a Diamonsil C18 (4.6 mm×250 mm, 5 μm) column with methanol-acetonitrile-water (containing 0.06% sulfuric acid, 0.5% sodium sulfate and 0.02% sodium heptanesulfonate) (16:16:68) as mobile phase, detection wavelength was 224 nm.To ensure a complete activation and condensation reaction, by means of moisture control within 0.5% and purity control not less than 98% for activator, 5-ethylthio-1H-tetrazole, were useful in the processing of synthesis.The methods are accurate and the limits are reasonable, had been used in tracking down the quality of 5-ethylthio-1H-tetrazole in oligonucleotide synthesis event.