The cold shield defines the angle of view field and limits stray radiation in cooled infrared imaging system. The cold shields have various aperture types but no analytical formula is available for cold shield effectiveness, and numerical methods are usually needed for its computation. A Monte Carlo method with MATLAB for computing cold shield effectiveness of a simplified round cold shield structure is presented. It is based on the concept of direction numbers in three dimensional space. First, a random point on cold shield is generated with MATLAB. Then a group of random direction numbers is associated to that point so an equation of a random straight line is formed. After tracing the ray reflected from the cold shield and judging that if it falls into the region of detector chip according to the property of specular reflection and formulae in analytical geometry, the cold shield effectiveness can be determined. It is easy in programming and convenient in operation.