Abstract:This paper proposes a new index (G(d)) based on the Ripley′s. The original Ripley′s index K(d) (here d is a distance scale) is an area estimate of a circle with radius d under the assumption that individuals have a Poisson distribution. The fact that an area estimate becomes larger when d gets larger makes it inconvenient for practical applications. The modified Ripley′s index L(d) transforms K(d) to an estimate of d and then d is subtracted from the estimate. The difference has an expectation of zero under the assumption of Poisson distribution. Although the use of L(d) is more convenient than K(d), the L(d) is still a number with a distance measure and not very convenient for use because its upper and lower envelopes look like a bell-mouthed form. The G(d) index proposed in this paper is a ratio with an expectation of zero and with no dimension and is defined as G(d) = K(d) / (πd 2 )- 1 Three examples with simulations and one real application show that G(d) has the property of stability, which previous Ripley’s indices (i.e., L(d) and K(d)) do not have, and is able to distinguish different spatial patterns. When the scale d reaches a certain value, such as between 2 and 6 in most cases, the envelopes become constants. Further analyses show that there is a relationship between the constants and the density of individuals (number of individuals per 100 m2). The relationship can be expressed as equations, which the stable values of upper-( SVU) and lower- (SVL) envelopes are as follows: SVU = 0.152549 - 0.0396694 ln(density), r2 = 0.919 SVL = - 0.175449 + 0.0610485 ln(density), r2 = 0.883 The results provide a possibility that it is not necessary to calculate the envelopes beyond the stable point if G(d) is employed.