You are not using the same basis in the log and pow
BigInteger bigInt = 10000000000000000000; // myBigInt is a huge BigInt i want to find the Log of.
double log;
log = BigInteger.Log(bigInt, 1000); //Find the Log
System.Diagnostics.Debug.WriteLine(log.ToString());
System.Diagnostics.Debug.WriteLine(((Int32)log).ToString());
BigInteger bigIntRecreate;
// unless log is an integer value it will round down and will not recreate to proper value
bigIntRecreate = BigInteger.Pow(1000, (Int32)log);
System.Diagnostics.Debug.WriteLine(bigInt.ToString("N0"));
System.Diagnostics.Debug.WriteLine(bigIntRecreate.ToString("N0"));
System.Diagnostics.Debug.WriteLine((bigInt - bigIntRecreate).ToString("N0"));