Current diffusion is an old issue, nevertheless, the relationship between the current diffusion and the efficiency of light emitting diodes (LEDs) needs to be further quantitatively clarified. By incorporating current crowding effect (CCE) into the conventional ABC model, we have theoretically and directly correlated the current diffusion and the internal quantum efficiency (IQE), light extraction efficiency (LEE), and external quantum efficiency (EQE) droop of the lateral LEDs. However, questions still exist for the vertical LEDs (V-LEDs). Here firstly the current diffusion length L s(I) and L s(II) have been clarified. Based on this, the influence of CCE on the EQE, IQE, and LEE of V-LEDs were investigated. Specifically to our V-LEDs with moderate series resistivity, L s(III) was developed by combining L s(I) and L s(II), and the CCE effect on the performance of V-LEDs was investigated. The wall-plug efficiency (WPE) of V-LEDs ware investigated finally. Our works provide a deep understanding of the current diffusion status and the correlated efficiency droop in V-LEDs, thus would benefit the V-LEDs' chip design and further efficiency improvement.