The development of large scale separation is critical to further advancement in the use of mini-hydrocyclones, which are proposed as promising equipment for high precision separation. But the key problem addressed in previous literatures was the non-uniformity of distribution since their easy sensitivity to variations of geometrical parameters and flow conditions. In this paper, the uniform distribution for UU-type parallel mini-hydrocyclones was further investigated with a theoretical study. Firstly, a general mathematical model was established with intake and underflow headers to simplify preliminary design of complex manifold systems. Developments of models and methodologies of solutions were compared between intake-overflow model and intake-underflow model. It was found that two models can be described by the same characteristic control equations. Therefore, the unity of two theoretical models was convinced. In addition, four characteristic parameters (D-c/D-i, n, Delta X and xi) are selected to analyze the parameter sensitivity, meanwhile discuss their effects on the uniformity of distribution. Results showed that the non-uniformity of distribution became larger with an increase of D-c/D-i, n or Delta X but xi oppositely, and that a proper selection does improve uniformity after other varying parameters were fixed. This theoretical study proposed a reasonable optimized scheme for uniform distribution during preliminary design of complex manifold systems, which provided convenience and flexibility for designers. (C) 2014 Elsevier B.V. All rights reserved.