Thickness of emissive layer in organic electroluminescent device is one of the important factors affecting the device performance. In this report, a blue electroluminescent device with an active layer of 4, 4'-bis(2, 2'-diphenylvinyl) -1, 1'- biphenyl (DPVBi) is fabricated. The device performance varies with the thickness of DPVBi. With the increase of the DPVBi thickness between 1050 nm, the device luminance and efficiency at the same current density first increase and then decrease, the device with a DPVBi thickness of 40 nm exhibits the highest luminance of 15840 cd/m2 and a maximum external quantum efficiency of 3.2%, with Commission Internationale de l'Eclairage (CIE) co-ordinates being (0.15, 0.15). The luminescent spectral red shift and the color purity deteriorate when the thickness is over 40 nm, which can be attributed to a result of microcavity effect. In the meantime, the analysis from experimental results shows that the exciton diffusion length in DPVBi is between 2030 nm.