Chemical force microscopy (CFM) was used to measure the adhesion force between quinolyl group (–Q)-terminated Au-coated tip and gold substrate (abbreviated as –Q/–Q) modified with self-assembled monolayers (SAMs) of 6-(10-mercaptodecaoxyl)quinoline (MDQ) in pH buffer solution. From the plot of adhesion force vs. pH value, which is called force titration curve, the pK1/2 of surface –Q group of MDQ SAM can be estimated to be 5.3, which corresponds to the pH value at the peak. By using the –OH group as a general probe, the similar pK1/2 value, estimated from the force titration curve of –OH/–Q and –Q/–OH, which also corresponds to the pH at the peak, is 5.2 and 5.4, respectively. From the conventional contact angle titration, pK1/2=5.9 was obtained. The difference of the obtained pK1/2 values arises from two different measurement methods and their measurement accuracies.