To analyze the root causes and reduce defects in precast concrete production by using quality tools and techniques including brainstorming, graph, pareto chart, cause and effect diagram, tree diagram, FMEA and control chart that following DMAIC of six sigma methodology. The methodology was composed of the five phase: (I) Define phase identified seven defect problems which were an incline block out, a nicked block out, a wrong length product, an on-side rough, a nicked key joint, a misaligned sleeve and quick tapping, (II) Measure phase measured defect proportion and defect per units that its trend prior improvement had continuously increased, (III) Analyze phase revealed root causes which were lack of training, the improper method, the deteriorate Jig, dirty and ruin materials and often breakdown machines, (IV) Improve Phase implemented the improvement actions to solve the problem causes with workers training, designing of new proper method, installing of crane smoothening machine and providing of new Jigs and maintenance plan, (V) Control phase used monitoring with control chart and then evaluated value of the improvement actions. The result shown significant reduction of all selected defects in the defective proportion and defect per unit which resulted in a total defective proportion of case factory reduces from 48.16% to 28.87%. A total defective proportion of PCF4 reduced from 32.90% to 12.30% and PCF1 reduced from 55.89% to 15.45%.
