The purpose of this study is probabilistic safety assessment of two different designs for emergency core cooling system (ECCS) in a typical PWR. The first design which is indeed the current operating design for ECCS includes i) high pressure safety injection system which comprises two identical trains that each of them includes two 50% capacity pumps, ii) low pressure safety injection system comprises two identical trains that each of them includes one 100% capacity pump, and iii) Two 100% capacity passive accumulators. The second design, in other words, the improved design namely, safety injection system, includes two trains that each of them comprises one 100% capacity pump and four 50% capacity passive accumulators. The assessment of two designs was completed in four steps. First, identify system components. Second, do failure mode and effect analysis. Third, perform fault tree analysis considering uncertainty and common cause failure and finally, calculate the core damage frequency via event tree analysis. By considering large break loss of coolant accident as an initiating event the final value of core damage frequencies are obtained 6.5 x 10-9/Reactor.Yr and 2.69 x 10-9/Reactor.Yr for the first and second design scheme of ECCS, respectively. It is found that there is a significant reduction in core damage frequency utilizing the second design for ECCS. All calculations are carried out by using Risk Spectrum software. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.