There are several aspects and criteria that should be considered for designing Feistel block cipher, including: block size, key size, number of rounds, subkey generation algorithm, round function, fast software encryption/decryption, and ease of analysis. The function F is the heart of Feistel block ciphers. It provides confusion property that makes the relationship between ciphertext and encryption key as statistically complex as possible. One obvious property is that F must be nonlinear. The more non linear F, the more difficult any type of cryptanalysis be. KASUMI is the Feistel block cipher that used in UMTS, GSM, and GPRS mobile communication systems. F-function component of KASUMI composed of FI, FL, and FO functions. In this paper, we analyzed the diffusion of F-function of KASUMI to determine its cryptographic strength using Strict Avalanche Criterion (SAC) and Bit Independence Criterion (BIC). The SAC test result showed that FI-subfunction has smallest relative error. Whilst, the BIC test results show that FL-subfunction has a zero value.