Prediction of surface errors in milling of thin-walled structures is not trivial due to cutting force induced deflections of workpiece. The situation becomes more complicated when the end mill adds cutter deflection errors to workpiece deflection. It is important to predict and analyze such variation of surface error from process planning and process control point of view. The study of surface error is also essential while overcoming the same by applying various compensation methods. This paper investigates cutter and workpiece deflection induced surface errors in peripheral milling of thin-walled straight and curved geometries. A flexible tool-workpiece system has been developed to estimate surface error in the presence of both tool and workpiece deflections. The effects of chip load and workpiece curvature on surface errors have also been investigated by using experimental and computational methodology. Based on the outcomes of the present study, it is concluded that the complex nature of tool and workpiece deflection induced surface error can be predicted both qualitatively and quantitatively in thin-walled machining. The results presented here provide useful insights into qualitative and quantitative nature of surface errors which will be helpful for product designers as well as process planners in improving machining productivity without sacrificing quality.