Objective:To investigate the influence of the depth of skin press in blunt tip placebo acupuncture needles on patient blinding and its relationship to needle diameter.Methods:Forty healthy volunteers were enrolled as subjects for patient blinding.Four acupuncturists applied the following needles randomly at three points in each forearm:0.18 mm and 0.25 mm diameter penetrating needles inserted to a depth of 5 mm,and 0.18 mm and 0.25 mm diameter skin-touch needles depressing the skin at the acupoint to a depth of 1 mm and 2 mm from the skin surface.The subjects reported their guesses at the nature of needles they received,and rated needle pain and de qi.A blinding index was calculated to define the success of blinding for subjects.Results:The blinding status of subjects for 1 mm press needles of 0.18 mm diameter was random guess,but unblinded for 1 mm press needles of 0.25 mm diameter.For 2 mm press needles of both diameters,the blinding status was opposite guess and the blinding status for penetrating needles of both diameters was unblinded. The percentages of felt pain with 2 mm press needles of both diameters were similar to that with penetrating needles,but those were not similar for 1 mm press needles.The frequency of de qi occurrence with 2 mm press needles of 0.18 mm diameter was similar to that of penetrating needles of both diameters.Conclusion:Placebo needles of 2 mm press made more subjects guess that the needles penetrated the skin than 1 mm press needles.The use of small diameter needles increased patient blinding.