A log-logistic probability of ignition (POI) model for ruptures of onshore natural gas transmission pipelines is proposed in this paper. The parameters of the proposed POI model are evaluated based on a total of 188 rupture incidents that occurred on onshore gas transmission pipelines in the U.S. between 2002 and 2014 as recorded in the pipeline incident database administered by the Pipeline and Hazardous Material Safety Administration (PHMSA) of the U.S. Department of Transportation. The product of the pipe internal pressure at the time of rupture and outside diameter squared is observed to be strongly correlated with POI and therefore adopted as the sole predictor in the POI model. The maximum likelihood method is employed to evaluate the model parameters. The 95% confidence interval and upper confidence bound on the POI model are also evaluated. The model is validated against an independent set of rupture incident data reported in the literature. The proposed POI model will facilitate the quantitative risk assessment of onshore natural gas transmission pipelines in the U.S.