For the missing carbon sink problem

terrestrial ecosystems are recognized as the biggest unknown field. Thus

to determine the role of terrestrial ecosystems in the global carbon cycle and understand its short-and long-term dynamics is a matter of high practical and scientific importance.Remote sensing has been attached increasingly more attention regarding the regional or global scale net primary production（NPP）estimation.The light-use efficiency model elaborated by Monteith has been widely used to estimate NPP based on remotely sensed data.In those NPP models based on remotely sensed images

NPP can be represented as the light-use efficiency multiplying absorbed photosynthetically active radiation.The absorbed photosynthetically active radiation is linear to the fraction of incident photosynthetically active radiation absorbed by vegetation（FPAR）.So FPAR is a very important parameter in the NPP model.For the exist- ing remote sensed NPP models

FPAR is calculated respectively from the Normalized Difference Vegetation Index（NDVI） used in Glo-PEM NPP model and Enhanced Vegetation Index（EVI）in VMP model

so it is influenced by VI characteris- tic property

which VI varies with view angle and bands of sensors.In fact

According to its definition

FPAR only depends on the solar incident angle and the canopy parameters.In this paper

Monte Carlo algorithm will be used to simu- late the radiance transfer process in canopy.In this paper

the spherical distribution of leaf angle distribution is taken as an example.On the one hand

we can get the function of bidirectional reflectance distribution at different wavelength

which can be used to calculate the vegetation index

such as Normalized Difference Vegetation Index（NDVI）used in Glo- PEM NPP model and Enhanced Vegetation Index（EVI）in VMP model.So the VIs can be used to study the property of FPAR value applied in Glo-PEM and VMP model.On the other hand

the Monte Carlo algorithm can be directly applied to simulate the fraction of active radiance absorbed by canopy

which is the FPAR.After the simulation of FPAR at differ- ent wavelength between 400-70Ohm

an averaged FPAR is integrated.It has been proved that the FPAR is function from solar zenith angle for the research of the leaf area index and the leaf angle distribution.Our simulated results show that the FPAR increases with the solar zenith angle

but the increasing range of FPAR depends on LAI.But when the LAI is up to 5

the FPAR is getting saturation and the FPAR changes smoothly with different solar zenith angles.The simulated results also show that the FPAR variations with LAI depend on solar zenith angle.For example

when solar zenith angle is set at a low value I0

the FPAR is saturated while LAI is up to 7.But when solar zenith angle is set at a big value

for example 70 degree

FAPR is easily saturated when LAI is getting to 3.Our simulated results also show FPAR does not dependent on view angle.It is consistent with definition of FPAR

which is defined as the fraction of incident solar energy between wavelength of 400 nm and 700 nm absorbed by canopy.