As device production is performed towards limits of k1 process, many issues are caused by lowering k1 value, which has been considered negligible at higher k1 value. Among these passed-over problems, illumination control error such as non-telecentricity is currently investigated in-depth. Comparing with projection lens, illumination system is not well verified and the amount of aberration is quite larger. Consequently, illumination pupil-fill has different shape along field position resulting in in-field distribution with a degree, which may lead to considerable critical dimension (CD) difference. Therefore, the effect of errors in illumination system should be evaluated to determine the required controllability of illumination system for low k1 imaging. Illumination control error is represented by the deformation and movement of the intensity profile in pupil plane and it can be decomposed into blurring, intensity unbalance and telecentric error or axis tilt, and so forth. In this paper, these components are computationally modeled and the modeling is implemented using an in-house lithography simulator. Using the modeling, the functionality of each illumination error component on CD variation can be separately resolved. The analysis results presented in this paper provide the relationship between CD control and required illumination control, and the allowable amount of control error for illumination can be estimated.