This paper presents an analysis of the relationship between the saturated hydraulic conductivity and the particle size of soils ranging from the coarse- to fine-grained ones. Based on the most widely used Kozeny-Carman (KC) model, a concept of equivalent particle size (or equivalent particle radius) considering adsorbed water film is introduced to derive a new simple hydraulic conductivity prediction model. This model is evaluated by using experimental data from this study and existing literature, and demonstrates satisfactory predictive capability for a wide range of soils. Moreover, this article discusses the reasons why the KC model is not applicable for predicting the hydraulic conductivity of fine-grained soil, and proposes a criteria in terms of critical particle size for determining the applicability of the KC model. The critical particle size is 1.0 × 10-3 mm determined by both the numerous experimental data and the comparison between the KC model and the proposed model. It further provides a threshold particle size of 1.0 × 10-5 mm as the minimum particle size for pore water flowing among soil particles.