Hello Student,

As you haven't mentioned for which stream you are talking about, I'll mention all the competitive examinations in science field
1.Civil services examination
This is most popular exam in India,one can give after graduation.
Graduates from any stream can apply for this exam.
2.IIT JAM
To fulfill the IIT dream after Bsc,you can apply for this exam to get admission for MSc in IISc, IITs, IISER, NITs and other CFTIs.
3.GATE
(Engineering graduates are eligible to apply for this exam)
4.UPSC indian engineering services examination
Engineering Graduates can appear for this exams
5 . Bank PO examination
(Science stream student can apply for banking exams also)
6.Indian economic service examination.
Students having a Bachelor’s Degree in Economics/Statistics can apply for the same.

See,these are some exams in science field.
Hope this helps
Good luck!!!