The study explain the use of DMAIC methodology of six sigma to minimize the defect rate in Coffee Processing and Warehouse Enterprise (CPWHE) Addis Ababa .It is a systematic approach towards defects minimization through five phases of DMAIC methodology which focuses on reduction/elimination of nine basic types of defects such as black, pest damage, white, green grass, immature bean, soiled bean, foxy, broken, brown and this defect affect process quality. Different six sigma tools were used in different phases. The define phase rolls out the tools such as SIPOC which clearly show the process map that includes suppliers, inputs, outs, customers and based on this the entire quality is control. In the measure phase, process capability was calculated. In the analyzing phase, the root causes of those defects were detected by cause and effect analysis .Pareto analysis was done to identify the major types of defects and the vital root causes that impact the defects by using regression, ANOVA, why-why. The improve phase concentrates on optimizing the vital root causes which impact the defects with their factors and levels, results of experimentation had been analyzed by using the above method for studying the variation and where does the variation came from? In control phase, control chart has been used to control the variation within acceptable levels in the process. Finally some potential solutions are suggested to overcome those causes. The result of the proposed solutions of the defect level in the section has been reduced as well as the result found to be very significant because Green bean coffee samples evaluation and grading for both raw (40%) and liquor (60%) quality was carried out for per each samples following the procedures of CLU (Coffee Liquoring Unit) (2007) as indicated. The overall standard for raw and liquor quality the average amount of pest damage before optimization where 4.61452 pes and has been reduced to 2.51852pes after optimization. At the same time the prediction value of black and immature beans also reduced from 12.8125pes to 1.71258 pes as the result from the prediction shows. Keywords: coffee process, Defects, Rework, DMAIC, ANOVA, SIPOC DPMO, Cause and Effect diagram, Pareto analysis