The effect of a short term temperature change on organic carbon oxidation, nitrogen removal and membrane permeate flux in a membrane separation bioreactor was studied. The process was developed as a household wastewater treatment unit, employing direct immersion of hollow fiber membrane modules for solid-liquid separation in an activated sludge aeration tank. The organic carbon oxidation in the system was not deteriorated by a short term temperature change whereas nitrogen removal decreased significantly as the temperature dropped to 10-degrees-C. Reduction in the percentage of strict aerobic bacteria suggests that limitation of oxygen transfer probably resulted in the decrease of the amount of nitrifying bacteria and, thus, inhibition of nitrification at low temperature. A temperature decrease affected the permeate flux not only by increasing the viscosity of mixed liquor but also changing the properties of the cake layer, i.e. cake layer thickness and/or porosity. An hysteretic curve of gel and cake layer was used to show the effect of temperature changes on the resistance of gel and cake layer. By differentiating the resistances of the porous cake layer from those of the gel and the extracellular matrix, changes in the cake layer thickness and/or porosity of the cake layer could be used to explain the changes in the cake layer resistance.