Experimental and analytical study is carried out on the stability of boring operation with special reference to the increase of stability by partly cutting off the side faces of a boring bar. The experimental results show the improved stability for two specific regions of the tool attachment angle defined between the directions of the cutting tool and the normal axis to the cut-off faces of a boring bar. The results also show that the degree of improvement is rapidly reduced for one of the regions with increasing the cut-off amount for the boring bar, though the stability in the other region is increased still more. Analysis is carried out based on the general theory of three dimensional chattering in three different ways depending on how strong hypotheses are imposed on the derivations of the simplified characteristic equations for the system. The first most simplified way is used to clarify the mechanism of improvement of sta-bility by the proposed method and the other two are for checking how accurately the experimental results can be explained by the analysis. Good agreements are obtained between the experimental and analytical results to show the validity of both the analysis and proposed method itself.