Improved approach to low-loss and high-uniformity multimode interference (MMI) devices is proposed in this paper. An optimized index contrast is selected to decrease the propagation constant errors of modes near cutoff in the MMI region. 1xN MMI splitters are analyzed by exact mode analysis. Results show that the loss and uniformity of this new structure are notably improved, especially when N is large. The loss of 0.0892 and uniformity of 0.0826 are acquired when N=32. A formulating approach for device-design is presented and it is suitable for all types of MMI couplers.