As specified in javadoc, a BigDecimal is defined by an integer value and a scale.

The value of the number represented by the BigDecimal is therefore (unscaledValue × 10^(-scale)).

So BigDecimal("1761e+5") has scale -5 and BigDecimal(176100000) has scale 0.

The division of the two BigDecimal is done using the -5 and 0 scales respectively because the scales are not specified when dividing. The divide documentation explains why the results are different.

divide

public BigDecimal divide(BigDecimal divisor)

Returns a BigDecimal whose value is (this / divisor), and whose preferred scale is (this.scale() - divisor.scale()); if the exact quotient cannot be represented (because it has a non-terminating decimal expansion) an ArithmeticException is thrown.

Parameters:

divisor - value by which this BigDecimal is to be divided.

Returns:

this / divisor

Throws:

ArithmeticException — if the exact quotient does not have a terminating decimal expansion

Since:

1.5

If you specify a scale when dividing, e.g. dividendo.divide(BigDecimal.valueOf(1000), 0, RoundingMode.HALF_UP) you will get the same result.

public static double divide(double d1, double d2) {
    BigDecimal b1 = new BigDecimal(d1);
    BigDecimal b2 = new BigDecimal(d2);
    return b1.divide(b2, 2, RoundingMode.DOWN).doubleValue();
}

You haven't specified a scale for the result. Please try this

2019 Edit: Updated answer for JDK 13. Cause hopefully you've migrated off of JDK 1.5 by now.

import java.math.BigDecimal;
import java.math.RoundingMode;

public class Main {

    public static void main(String[] args) {
        BigDecimal a = new BigDecimal("1");
        BigDecimal b = new BigDecimal("3");
        BigDecimal c = a.divide(b, 2, RoundingMode.HALF_UP);
        System.out.println(a + "/" + b + " = " + c);
    }

}

Please read JDK 13 documentation.

Old answer for JDK 1.5 :

import java.math.*; 

    public class x
    {
      public static void main(String[] args)
      {
        BigDecimal a = new BigDecimal("1");
        BigDecimal b = new BigDecimal("3");
        BigDecimal c = a.divide(b,2, BigDecimal.ROUND_HALF_UP);
        System.out.println(a+"/"+b+" = "+c);
      }
    }

this will give the result as 0.33. Please read the API