The ZUC algorithm is a new stream cipher, which is the core of the standardised 3GPP confidentiality and integrity algorithms 128-EEA3 & 128-EIA3. In this paper, we analyze the initialization stage of ZUC v1.5. First of all, we study the differential properties of operations in ZUC v1.5, including the bit-reorganization, exclusive-or and addition modulo 2
, bit shift and the update of LFSR. And then we give a differential trail covering 24 rounds of the initialization stage of ZUC v1.5 with probability 2
, which extends the differential given in the design and evaluation report of ZUC v1.5 to four more rounds. Nevertheless, the study shows that the stream cipher ZUC v1.5 can still resist against chosen-IV attacks.