Murine models are commonly used to investigate bone healing and test new treatments before human trials. Our objective was to design an improved murine femur fracture device and determine optimal mass and velocity settings for maximal likelihood of transverse fracture. Fracture reproducibility was maximized using an adjustable kinetic energy level, a novel mouse positioning system and an electromagnet striker release assembly. Sixty wild-type mice of 8-12-week-old male and female with a weight of 26.4+/-6.1g were subjected to an experimental postmortem fracture in the left and right femur (n=120) using variable kinetic energy inputs. A best-fit prediction equation for transverse fracture was developed using multivariate linear regression. Transverse fracture was shown to correlate most highly with kinetic energy with a maximum likelihood at mv2=292 where m is mass (g) and v is velocity (m/s). Model validation with a group of 134 anesthetized C57BL/6 mice resulted in a favorable transverse fracture rate of 85.8%. Simple modifications to existing fracture devices can improve accuracy and reproducibility. The results may assist researchers studying the effects of genetic modifications and novel treatments on boney healing in murine femur fracture models. Maintaining kinetic energy parameters within suggested ranges may also aid in ensuring accuracy and reproducibility.