The scuffing process was in-situ observed in a sliding contact between a rotating sapphire disc and a stationary steel ball under dry condition and lubricated condition. The in-situ observations indicate that under both dry condition and lubricated condition scuffing is initiated as a result of the accumulation by wear particles. To further understand the role of wear particles in scuffing initiation, wear particles and contact area from sliding tests terminated prior to scuffing initiation and also just after scuffing initiation were examined. The shape and composition of the wear particles both prior to and after scuffing initiation were obtained. The profile, composition, and micro-hardness of the steel surface prior to scuffing initiation were also obtained with special interests on properties of the particle agglomerates formed on the steel surface. The role of wear particles in scuffing initiation was discussed based on these observations.