【Objective】The phenomenon of aggregation of colloid particles varying from ion to ion the same in valence in solutions the same in electrolyte concentration is referred to as specific ion effect or Hofmeister effect. In this study, aggregation of montmorillonite colloid particles were observed under an atomic force microscope (AFM) and their adhesion force measured, in an attempt to analyze specific ion effect of the particles in aggregation and hence to provide experimental support to theoretical studies on interactions between ions and particles, and references for further studies on specific ion effects of aggregating particles in strong electric fields.【Method】The montmorillonite particles that had gone through a 200 mesh sieve were added into LiCl, KCl, NaCl, RbCl and CsCl solutions 10, 30 and 50 mmol L^-1 in concentration, separately, and prepared into montmorillonite suspensions, which were then dripped onto mica sheets, separately, air dried, and scanned with AFM. 【Result】 (1) With rising concentration of electrolyte in the system, aggregation of montmorillonite particles occurred horizontally first and then vertically. (2) In solutions varying in electrolyte, but the same in concentration, the montmorillonite particles aggregation degree exhibited obvious specific ion effects. The montmorillonite particles in all the electrolyte solutions 10 mmol L^-1 in concentration were all around 1 nm in height; in the Cs⁺ system, montmorillonite particles were about 300 nm in diameter, and aggregated horizontally, while those in the other ionic systems were merely 100 nm, and did not aggregate horizontally. In the electrolyte solutions 30 mmol L^-1 in concentration, Montmorillonite particles in the Rb⁺ system, they reached 400 nm in diameter and about 4 nm in maximum height, which indicates that the particles aggregated horizontally, and vertically, too, though not much, and in the Cs⁺ system, Montmorillonite particles did aggregate horizontally and vertically. And in the solutions 50 mmol L^-1 in concentration, Montmorillonite particles in the Li ⁺, Na ⁺, K⁺, Rb ⁺and Cs⁺ systems were approximate 0.7 、0.8、2、2~14 and 60, respectively, which indicates that Montmorillonite particles exhibited weak superposition in the K⁺ system, strong aggregation process in the Rb+ system with the particles vertically stacked up to 10 layers, and more significant aggregation in the Cs+ system. (3) In solutions with a given ionic concentration, the maximum adhesion force between Montmorillonite particles and the probe varied in the order of Li⁺ < Na ⁺ < K⁺ < Rb⁺ < Cs⁺, showing significant specific ion effects. The variation was slight when the electrolyte concentration of the solution was low, but great when it was high.【Conclusion】 Through direct AFM observation, it was found that strong specific ion effects existed in the aggregation of montmorillonite particles and varied in the sequence of Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺. The colloid particles aggregated mainly horizontally in solutions low in ionic concentration, and then began to do vertically with increasing ionic concentration. The higher the aggregation degree, the higher the adhesion force.