Abstract: The formation, extraction, classification and the characteristics of active groups of humic acids(HA) are discussed. HA affect many aspects of plant physiological activities, including plant growth, uptake of nutrient elements, synthesis of protein and DNA, photosynthesis, respiration and activities of some enzymes. The effects of HA on plants vary, depending on the component and contents of HA, the species of plants and the nutrient conditions of environment. The mechanisms of HA increasing plant growth and respiration and affecting uptake of nutrient elements also are discussed at cellular or molecular levels.