The quality of the software's internal structure tends to decay due to the adaptation to environmental changes. Therefore, it is beneficial to maintain the internal structure of the software to benefit future phases of the software life cycle. A common correlation exists between decaying internal structures and problems like software smell and maintenance costs. Refactoring is a process to maintain the internal structure of software artifacts based on the smell. Decomposition of classes is one of the most common refactoring actions based on Blob smell performed at the source code level. Moving the class decomposition process to the design artifact seems to affect the quality and maintainability of the source code positively. Therefore, studying the impact of design-level class decomposition on source code quality and software maintainability is essential to ascertain the benefits of implementing design-level class decomposition. The metricsbased evaluation shows that the design-level class decomposition positively impacts the source code quality and maintainability with the rank biserial value is 0.69.