Micro-phase separation in binary mixed Langmuir monolayers of cadmium salts of n-alkyl fatty acids (CH3(CH2)(n-2)COOH; Cn (n = 18, 20, 22, 24)) and a perfluoropolyether surfactant (F(CF(CF3)CH2O)(3)CF(CF3)COOH, PFPE) is studied by film balance measurement and atomic force microscopy (AFM). At different temperatures, mixtures of Cn/PFPE in chloroform were spread onto the aqueous Cd2+ subphase and deposited on silicon wafers. AFM images showed that Cn and PFPE separate into microscopic domains of condensed phase and a surrounding matrix of expanded phase, respectively, in their mixed monolayers. The morphological feature of phase-separated structures was characterized by characteristic length (lambda) expressing the periodicity of two-phase distribution and fractal dimension (D) of Cn domains reflecting the complexity of domain shape, which were determined through AFM image analyses. It was found that the monolayer morphologies systematically vary with alkyl chain length of Cn and temperature of the water surface; circular-shaped condensed phase microdomains with D of about 1.1 are formed at lambda of 7-9 mum when the mixed monolayers are prepared using a shorter alkyl chain fatty acid and/or at a, higher temperature, whereas branched narrow domains with D close to 2 are formed at lambda of approximately 4 mum when a longer chain fatty acid is used at a lower temperature.