Zirconium is well known as an excellent grain refiner for magnesium alloys that do not contain aluminum.  Currently, approximately 1% by weight of zirconium is added to magneisum alloys for grain refinement.  In this work, a Design of Experiments (DOE) approach was used for a systematic study of the grain refinement of magnesium by zirconium.  Variables included the amount of zirconium, the pouring temperature, and the holding time prior to casting.  Samples were poured into a special “hockey puck” mold designed to reproduce the conditions in permanent mold casting.  Optical metallography and quantitative image analysis were used to measure the resultant grain sizes.  The effect of the various factors on the measured grain size and the interaction among the various factors is discussed.  An approach for the development of an efficient Zr-based grain refiner is outlined.