is the angle we get from top land of gear tooth differ from its pitch diameter.... i have a problem with the gear designed from china, modified, they seem to make pitch diameter bigger than usual,
The pitch diameter is between the tip and root diameters and cannot be measured on the gear. It is an artefact of meshing with another gear. The same is true of pressure angle. However, gears are commonly produced using standardized rack dimensions to ensure interchangeability. Definitions are here - http://en.wikipedia.org/wiki/Gear gears of a given pitch can be produced at a large range of pressure angles, but gears of different pressure angles will not mesh properly. Pressure angle is the angle at which the line of action intersects the pitch circle.
Pitch circle is an imaginary circle which occurs only when two gears are in mesh. From your description, I suspect that the gears are "Corrected". Gear Correction is done to improve either strength or to improve the contact ratio. For such gears it is important to measure the mating gears too. Measure the Center Distance. If it is the theoretical CD, then the gears are said to be S0 corrected, else, they are said to be S1 corrected. S1 correction is resorted to when the number of teeth on the smaller gear is < 17 for a cut pressure angle of 20 deg.