Abstract: Effect of osmosis onthe growth of and polysaccharide accumulation in theprotocorms of Dendrobium candidum on a culture medium were studied.The liquid medium wasadded with mannitolat varied concentrations to alter theosmotic pressure for the experiment. The results showed that,at low concentrations of mannitol, the growth andpolysaccharide synthesis ofthe protocorms were enhanced,but significantly inhibited at high concentrations. During the cultivation,the vigor, growth, polysaccharidecontent, andyield of the protocormsincreased when the mannitol concentration in the medium was 0-20 gL-1.After 35d culturein the medium with 20 gL-1 mannitol,the dry weight, polysaccharide content, andyield ofthe protocormspeaked with the maximized values of 17.30 gL-1, 361.52 mgg-1, and 6255.38 mgL-1, respectively. However,when the mannitol concentration was raised to 30-40 gL-1,all of those indices hit a downward trend suggesting the high osmotic pressure could be detrimental to theprotocorm cultivation.