A mathematical model based on both frequency and relative water content of two tropical crops' rubber and magnolia' leaves derived from s(21) measurements has been proposed in the frequency range of 37 GHz. Leaf samples sandwiched with Plexiglas side holders are inserted within waveguide sections, and the dielectric constants are calculated from the complex transmission coefficient S-21. In general, complex dielectric constant of materials (leaves in this case) is a function of frequency and water content. A model obtained from the data of rubber based on both frequency and relative water content is compared with the models in the literature and verified by measurements of control samples (magnolia in this case). From the comparisons, it is observed that proposed model has been found to show more promise in prediction of the dielectric constants, and model has, at most, +/- 10% error in relative dielectric constant (epsilon(r)) in broad band measurements. Relative dielectric constant is decaying by similar to 1/f(2) and shifts down by relative moisture constant by similar to M-r.