Quantitative, efficient, and accurate measurement of upper limb motor performance is relevant for monitoring upper limb progress in daily activities of people with cerebral palsy, which is helpful to define an appropriate rehabilitation procedure. Currently, motion capture sensors using a Natural User Interface (NUI) are the most feasible solutions due to the low-cost implementation, portability, and effectiveness for upper limb motion analysis. A quantitatively assessment study of the upper limb motor performance for children with spastic hemiparesis using the NUI Kinect v2 sensor is described in this paper. The study participants were eighteen children with an average age of 8.28 ± 2.32 years having a Manual Ability Classification System level I and II. The assessment was done for each participant before and after the application of a Modified Constrained-Induced Movement Therapy (mCIMT) applied five days a week along twelve weeks. According to obtained results significant differences (p<0.05) in the Fugl-Meyer Assessment (FMA) score for motor performance in abduction movement of upper limb between the first and last sessions were quantitatively detected. The described NUI assessment method helped to quantitatively show that the applied therapy sessions were effective to improve upper extremity motor performance. Additionally, the upper limb movements motorically limited after the rehabilitation therapy were also identified. The main contribution of the described NUI assessment method is its potential use as a quantitative measurement tool, which might be used by specialists to objectively diagnose and consequently define an appropriate rehabilitation therapy for patients with hemiparesis.