Thanks, guys. I change my code a bit. The results are right. But I suspect it never used thread, but excute the thread one by one. new code void mtread(char* filename,const int stidx, const int edidx,int id) { fstream infile; infile.open(filename); string line; int linenum = 0; if (infile.good()){ while (! infile.eof()) { boost::mutex::scoped_lock lock(io_mutex); // infile.seekg(linenum,ios::beg); getline(infile,line); if (linenum>=stidx && linenum<=edidx){ cout<edidx) break; } } } else { cerr<< "file " << filename << " cannot be opened for reading" <