In This paper some problems, which should be noticed in the measurements of organicmolecular first hyperpolarizability（β） by the solvatochromism , are discussed based on the resultsof the measurements for 11 compounds .The experiments indicate theat the selection of solvent used is the basis of the method to fit for use. The determination of the parameter a, which is the radius of a spherical cavity occupied by the molecules in the solutions, is the key to decide’the reasonableness of the obtained results and the formula for a=1/2(l’+rv),has been suggestedin which l is the molecular length of the nonlinear optical active part and can be obtained fromvalues of bond lengths in literature, rv is the average spherical radius of used solvents.
