Soil moisture measurement by the neutron probes is affected near the soil surface where neutrons are lost into the air. So, the usefulness of the neutron meter is limited if it is necessary to measure moisture near the soil surface. Also, most researchers need to know the shallowest depth at which moisture observations are unaffected by the surface. Monte Carlo simulation technique is a powerful tool to model the measuring moisture by neutron probe and calculate the surface effect. In this present work, moisture measurement by a neutron probe with Am-Be neutron source has been simulated with MCNP4C code. The dimension of a properly paraffin cube has been calculated to correct the surface effect and use neutron probe near the soil surface measuring.