The simplest positron-organic compound, the PsCH(3) system, has been calculated by quantum Monte Carlo (QMC) and nb initio molecular orbital (MO) methods. Tn the QMC method the Jastrow factor is added to the trial function to satisfy the cusp condition, and parameters of trial function are optimized by the variational Monte Carlo method. In the nb initio MO method the large basis set based on the conventional 6-31G is used and the Moller-Plesset second-order energy is included. The positron affinity is calculated to be 6.92 eV by QMC and 5.15 eV by ab initio MO, Using the value of 6.92 eV, we have found the binding energy between CH3 and Pa to be 0.19 eV, and using the value of 5.15 eV, we find the binding energy to be -1.58 eV. However, the possibility of getting a larger positron affinity by the nb initio MO method can be expected with a larger basis set, so the stable existence of the PsCH(3) system may be predicted by this QMC calculation.