datum curve

Pick one or both (planar?) curves and the Intersect tool (Edit / Intersect). Or, have you got that part down and having trouble with something specific?

In 2001 I create datum curve from 2 project curves on fly, so as you say I have to create 2 datum curves in advantge, and then using those 2 curves intersection ???? right now I am using one curve project on to the surface. but I could not do 2 curves intersection on fly. Henry

With a curve projected onto a surface, you have the effect, the equivalent of curve from two projections. The first curve is one projection, the surface extrudes from a curve on a plane normal to the first. Once locked to the surface, no more variation of the curve, or any point along it, is possible.

If David's response doesn't hit the mark, perhaps post a sample part file to mcadcentral.com and explain the situation. I'm not following what you are trying to do (the "on the fly" part, knowing nothing about 2001).