This study reports experimental results and correlations for axisymmetric gaseous fuel jets restricted by parallel sidewalls at various separation distances. Although many investigations have been conducted to elucidate the flame height evolution of diffusion flames in an unrestricted environment; the restriction effect of sidewalls on diffusion flames, which occasionally occurs in accidental leakages of city natural gas pipelines, has received little attention. The underlying interaction dynamics of axisymmetric gaseous fuel jets with two parallel sidewalls at various separation distances has not been fully elucidated. In this work, a series of experiments on this issue were carried out with 3-, 6- and 10-mm nozzles. The sidewall separation distances were varied from 10 to 50 cm with a corresponding free condition. A series of new results and their interpretation are presented in this work. The results show that the flame height changes little when the sidewall separation distance reduces from + infinity to a critical value (S-cn) Further reductions on the sidewall separation distance from S-cri disturbed the evolution process of uprising vortexes and hindered air entrainment, leading to significant changes to the jet-flame shape by enlarging the flame height. The maximum flame heights had a linear relation with the critical separation distance of the sidewalls at the critical conditions, being consistent with the scaling analysis of the flow field. The dimensionless critical separation distance was found to be well correlated with the dimensionless heat release rate, Q(D*), with a 2/5 power law. A global model, characterizing the variation of the flame height with the dimensionless heat release rate, was proposed, showing good agreement with the experimental results. The results and the expressions obtained in this study contribute to a better understanding of jet fires, allowing a better prediction of flame height, relevant to the design of gas fuel storage systems and transportation systems in the city.