SNOW3G stream cipher is the core of the standardized 3G Partnership Project (3GPP) confidentiality and integrity algorithms UEA2 & UIA2 while ZUC stream cipher is the core of the standardized 3GPP confidentiality and integrity algorithms 128-EEA3 & 128-EIA3. So far, there have been no Guess and Determine attacks applied to SNOW3G. In this paper, a Guess and Determine attack on SNOW3G is proposed with a computational complexity of 2³²⁰, requiring 9 keystream words (each word consists of 32 bits). After analyzing the design of ZUC, a half-word-based Guess and Determine attack on ZUC is introduced, based on transforming the word-based nonlinear function of ZUC into a half-word-based nonlinear function. The attack on ZUC has a computational complexity of 2³⁹² and requires 9 keystream words, which is better than the previous Guess and Determine attack on ZUC. These results show that ZUC has much better resistance against Guess and Determine attack than SNOW 3G, though the internal state size of ZUC is smaller than SNOW 3G.