The purpose of this study is to attain a state of ultrahigh vacuum (UHV) under a mild baking condition using chemical treatment. The vacuum test involved the study of an aluminum alloy chamber 8.5 m long, 2 m(2) in surface area and 27 rho in volume with an elliptical aperture for the Recycler Ring at Fermilab. After the chamber was exposed to enough air of 95% relative humidity and evacuated to 10(-5) Torr, dichloropropane (DCP) gas of 1 Torr was injected into the mildly heated chamber (at 80 degrees C) and evacuated. This process was repeated between three and four times, depending on the type of test performed. After the first chemical treatment (test-1 DCP), the ultimate pressures were 7 x 10(-10) Torr at the left end and 9 x 10(-10) Torr at the right end, an only slight difference. When test-1 DCP reached the ultimate lowest pressure, the chemical retreatment (test-2 DCP) was applied under the mild baking condition on the entire inner surface of all components : chamber, Bayard-Alpert gauges, a quadrupole mass spectrometer, an ion pump, and a turbomolecular pump. Test-2 DCP showed that the ultimate pressure at the left end near the main pump was less than 1 x 10(-11), which was under the detectable limit of the gauge. On the other hand, in the case of a reference test (test-1 ref) conducted under the same condition as test-1 DCP but without the chemical treatment, the ultimate pressures were twice as high at the left end and 80 times higher at the right end than the pressures of test-1 DCP. Therefore, it was assumed that the lower outgassing rate resulted from the chemical treatment. In addition, it was suggested that DCP has a potential to promote water evacuation by breaking a weak intermolecular hydrogen bond of adsorbed multilayer water which is preferentially dissociated rather than a strong intramolecular oxygen-hydrogen bond of water.