Background: Psoriasis is a major common and relapsing disease which is both physically and psychologically disabling.Narrow band UVB (NB-UVB) is a new phototherapy option for psoriasis due to its easy administration and less acute and remote hazards than other forms of phototherapy.Aim: In this study, a double half bilateral comparative trial on 20 patients with chronic plaque psoriasis was carried out to compare suberythemogenic dose of NB-UVB versus erythemogenic dose in the treatment of psoriasis.Patient and method: Our study was conducted on 20 patients with chronic plaque psoriasis. Half body UV exposure was achieved by covering half of the body with an opaque overall cut in half length ways.The left body side was treated with the dose causing minimal erythema (100% of MED) while the right side was given 70% of this minimal erythema dose (suberythemogenic side).Results: Our results revealed no statistically significant difference in PASI final and in the percentage of reduction of PASI score between both sides as well as the total number of sessions (P-value > 0.05) while our most significant difference was the lower total cumulative UVB dose on the suberythemogenic side (P-value < 0.001).Conclusion: Our study recommends reducing the dose regimen of NB-UVB and consequently the cumulative UVB dose by using the suberythemogenic dosing schedule as it does not only affect the therapeutic potential of NB-UVB, but also improves the benefit/risk ratio of NB-UVB therapy.