Effect of NH acidity on transfer hydrogenation of Noyori-Ikariya catalyst RuH(TsDPEN)(p-cymene) has been studied based on a series of tosylated ethylenediamine ligands (TsEN-Ph-X). Initial rates studies in both 2-propanol and 5:2 formic acid/triethylamine azeotrope show that catalysts containing less acidic NH groups afford better catalytic reactivities. The correlation of catalytic reactivities with the pKas of the NH groups shows a linear pattern in 2-propanol but a concave downwards relationship in azeotrope. These results lead to better understanding of the mechanism of Noyori-Ikariya's catalyst and also aid in designing superior catalysts.