This study was conducted during eight years to determine the self-thinning exponent α in overcrowded Kandelia obovata stands in Manko Wetland, Okinawa Island. Using the ordinary least squares (OLS) regression, the α-value based on the self-thinning equation w = K •ρ ^-α (Yoda et al. 1963) was 1.348 ± 0.641, which was significantly different from 3/2 (p < 0.05) and insignificantly different from 4/3 (p > 0.05). Using the standard major axis (SMA) regression, the α- value was 1.461 ± 0.641, which was insignificantly different from 3/2 (p > 0.05) and significantly different from 4/3 (p < 0.05). In the allometric model 1/ρ α w^ф , the allometric coefficient ф was 0.6310 ± 0.0192, i.e. α = 1/ф = 1.585 , which was insignificantly different from 2/3, i.e. 1/ф = 3/2 (p > 0.05) and was significantly different from 3/4 (p < 0.05). In case of Weller’s extension of the allometric model, α ( = 1/ф = 1/{1-( θ + δ)}) was calculated to be 1.585 from the allometric exponent θ (= 0.3857 ± 0.01452) of the relationship between mean tree height H and w , and the allometric exponent δ (= -0.01673 ± 0.01707) of the relationship between mean mass density d and w. The allometric exponent δ was insignificantly different from 0.0 (p > 0.05). The values of α were the same between the allometric model and its extension.