According　to　the　principle　that　the　contact　angle　of　liquid　droplet　always　increases　on　a　limited　liquid-solid　interface,　it　is　suggested　that　the　integration　of　many　small-size　limited　liquid-solid　interfaces　results　in　the　increase　of　the　hydrophobicity　of　lotus-leaf-like　micro-convex-concave　surfaces.　Mathematical　equations　of　the　stability　of　liquid-droplets　on　the　surface　of　lotus-leaf-like　structure　were　established.　The　relationship　between　the　theoretical　critical-radius　of　the　void　of　micro-convex-concave　surface　and　the　nature　of　the　solid　and　the　liquid　was　drawn.　The　three　conditions　of　realizing　hydrophobicity　were　described.　The　result　of　computation　has　shown　that　when　the　radius　of　the　void　of　micro-concave-convex　surface　is　less　than　the　theoretical　critical-radius　r　(c),　the　droplets　may　always　be　in　a　stable　state　on　the　solid　surface　with　the　contact　angle　greater　than　90A　degrees.　The　minimum　area　of　the　liquid-solid　interface　and　low　surface　energy　of　solids　are　important　factors　in　realizing　hydrophobicity.　The　effective　work　of　adhesion　W　(a)　＇　was　proposed　as　a　criterion　for　measuring　the　hydrophobic　ability　of　the　solid　surface.