Abstract: When a 100 g cone （apax angle 30°） penetro-meter is used to determine simultaneously the liquid and plastic limits of soil, the crucial problem is first of all how to determine precisely the penetration depth hp, at plastic limit, so that the plastic limit of that soil can be read out directly from the h-w straight line （relation between cone penetration depth and moisture content） on the log-log coordinates. It was proved by test results that the plastic limits of sandy soils determined by the hyperbolic curve method were a little bit lower than that obtained by the conven tional thread rolling method, however, based on demonstrations by test data of more than one hundred sandy soil samples, the plastic limits determined by the orthogonal polynomials curve or the combined curve （formed from both the hyperbolic curve and the orthogonal polynomials curve） were consistent with that obtained by the thread-rolling method. The hyperbolic curve method is rather feasible for silty and clayeyn soils. Thus the values of plastic limits determined by different curves respectively commensurate with different types of soils would become more precisely. Therefore,o the presence of orthogonal polynomials curve will make further improvement the combined determination method.