In this paper, a new conserved current for Klein-Gordon equation is derived. It is shown, for 1 + 1-dimensions, the first component of this current is non-negative and reduces to |φ| 2 in nonrelativistic limit. Therefore, it can be interpreted as the probability density of spinless particles. In addition, main issues pertaining to localization in relativistic quantum theory are discussed, with a demonstration on how this definition of probability density can overcome such obstacles. Our numerical study indicates that the probability density deviates significantly from |φ| 2 only when the uncertainty in momentum is greater than m0c.
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