Micro milling is widely used in precision manufacturing industry which is suitable for producing micro scale parts having three dimensional surfaces made from engineering materials. High material removal rate is its main advantage over other micro manufacturing technologies such as lithography, micro EDM, laser ablation etc. Modeling of micro milling process is essential to maximize material removal rate and to obtain desired surface quality at the end of the process. The first step in predicting the performance of micro milling process is an accurate model for machining forces. Machining forces are directly related to machine tool characteristics where the process is performed. The spindle and the micro milling tool affects the machining forces. In this thesis, the influence of runout of the spindle system on micro milling forces is investigated. Two different spindle systems with different levels of runout are considered and necessary modifications are introduced to model the trajectory of the tool center for better prediction of process outputs in the presence of runout. A modified mechanistic force modeling technique has been used to model meso/micro scale milling forces. Detailed micro milling experiments have been performed to calculate the cutting and edge force coefficients for micro end mills having diameters of 2, 0.6, and 0.4 mm while machining titanium alloy Ti6AL4V. Good agreements have been observed between the predicted and measured forces. It is found that statically measured runout values do not translate into dynamic machining conditions due to machining forces acting on the end mill.