This study compared photosynthetic characteristics on the tip,middle,base of juveniles Sargassum thunbergii.Parameters analyzed included:net photosynthetic oxygen evolution rate(P_n),dark respiration rate(R_d),rapid light curve and fluorescent dark relaxation parameters.This was determined using both a liquid-phase oxygen electrode system,and pulse amplitude modulated fluorometry.In addition,the growth rate of juveniles was measured by weight,and its relationship with photosynthesis was discussed.The formation of vesicles was observed.The results show that:(1)P_n and Rd in the tip were higher than those in the middle and base(P<0.05).(2)In the low light(LL)adapted samples there was a delay in photosynthesis,and PSⅡ was immediately inhibited if the samples were put in light conditions.However,medium light(ML)conditions showed photochemical reactions and heat dissipation to be in balance.In high light(HL)conditions electric transport rate(ETR)was near saturation,and non-photochemical quenching(NPQ)was higher than LL and ML conditions.It was also observed that under HL conditions ETR_max in tip and middle of the juveniles was higher than that in the base(P<0.05).(3)For LL and ML adapted samples,NPQ recovery was fast,whereas in HL it was slow.Under all light levels(LL,ML,HL)in the samples tip,NPQ recovered rapidly and completely to 0.This indicates highly efficient restoration in the tip,but not in the middle and base where photodamage may be occurring.(4)Under ambient light levels pigment content was higher in the tip than that in the middle and base,and Chlorophyll a contents were higher than carotenoid in the tip,which was contrary in the middle and base.(5)The specific growth rate values relative to position on the algae were as follows:tip >middle >base(P<0.05),indicating fast organic matter accumulation in the tip.It was observed that in increased temperature conditions,small gaps which formed in the narrow leaf tip developed into vesicles.This suggests that temperature is the main inducing factor for the formation of vesicles.The results of this study suggest that photosynthetic rate,growth rate,and recovery capability in the tip of juvenile S.thunbergii may be higher than those in the middle and base.