This study reports the experimental investigation on the flow imbalance phenomena during filling symmetrical multi-cavity in injection molding of optic elements. There are two aspects in this study: First, the flow imbalance in the eight-cavity mold with normal H-type symmetric runners were observed and measured with aid of short-shot experiments. Second, the effects of three processing conditions including injection speed, melt temperature and mold temperature on flow imbalance were investigated. Flow imbalances are consistently observed and quantitatively measured. The results have shown that proper injection speed, high melt and high mold temperature can reduce the flow imbalance.